Difference between revisions of "Btrfs"

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[[Category:File systems]]
 
[[Category:File systems]]
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[[fr:Btrfs]]
 
[[ja:Btrfs]]
 
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[[zh-CN:Btrfs]]
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[[ru:Btrfs]]
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[[zh-hans:Btrfs]]
 
{{Related articles start}}
 
{{Related articles start}}
 
{{Related|File systems}}
 
{{Related|File systems}}
{{Related|Btrfs - Tips and tricks}}
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{{Related|Snapper}}
{{Related|Mkinitcpio-btrfs}}
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{{Related|dm-crypt/Encrypting an entire system#Btrfs subvolumes with swap}}
 
{{Related articles end}}
 
{{Related articles end}}
  
From [[Wikipedia:Btrfs]]
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From [https://btrfs.wiki.kernel.org/index.php/Main_Page Btrfs Wiki]:
  
:''Btrfs (B-tree file system, variously pronounced: "Butter F S", "Better F S", "B-tree F S", or simply "Bee Tee Arr Eff Ess") is a GPL-licensed experimental copy-on-write file system for Linux. Development began at Oracle Corporation in 2007.''
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:Btrfs is a modern copy on write (CoW) filesystem for Linux aimed at implementing advanced features while also focusing on fault tolerance, repair and easy administration. Jointly developed at multiple companies, Btrfs is licensed under the GPL and open for contribution from anyone.
  
From [https://btrfs.wiki.kernel.org/index.php/Main_Page Btrfs Wiki]
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{{Warning|Btrfs has some features that are unstable. See the Btrfs Wiki's [https://btrfs.wiki.kernel.org/index.php/Status Status], [https://btrfs.wiki.kernel.org/index.php/FAQ#Is_btrfs_stable.3F Is Btrfs stable?] and [https://btrfs.wiki.kernel.org/index.php/Getting_started Getting started] for more detailed information. See the [[#Known issues]] section.}}
  
:''Btrfs is a new copy on write (CoW) filesystem for Linux aimed at implementing advanced features while focusing on fault tolerance, repair and easy administration. Jointly developed at Oracle, Red Hat, Fujitsu, Intel, SUSE, STRATO and many others, Btrfs is licensed under the GPL and open for contribution from anyone.''
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== Preparation ==
  
{{Warning|
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For user space utilities [[install]] the {{Pkg|btrfs-progs}} package, which is required for basic operations.
* Btrfs has some features that are considered experimental. See the Btrfs Wiki's [https://btrfs.wiki.kernel.org/index.php/Main_Page#Stability_status Stability status,] [https://btrfs.wiki.kernel.org/index.php/FAQ#Is_btrfs_stable.3F Is Btrfs stable,] and [https://btrfs.wiki.kernel.org/index.php/Getting_started Getting started] for more detailed information.
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* Beware of the [[#Limitations|limitations]].
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If you need to boot from a Btrfs file system (i.e., your kernel and initramfs reside on a Btrfs partition), check if your [[boot loader]] supports Btrfs.
 +
 
 +
== File system creation ==
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The following shows how to create a new Btrfs [[file system]]. To convert an ext3/4 partition to Btrfs, see [[#Ext3/4 to Btrfs conversion]]. To use a partitionless setup, see [[#Partitionless Btrfs disk]].
 +
 
 +
See {{man|8|mkfs.btrfs}} for more information.
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=== File system on a single device ===
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 +
To create a Btrfs filesystem on partition {{ic|/dev/''partition''}}:
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 +
# mkfs.btrfs -L ''mylabel'' /dev/''partition''
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 +
The Btrfs default blocksize is 16KB. To use a larger blocksize for data/metadata, specify a value for the {{ic|nodesize}} via the {{ic|-n}} switch as shown in this example using 16KB blocks:
 +
 
 +
# mkfs.btrfs -L ''mylabel'' -n 16k /dev/''partition''
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 +
=== Multi-device file system ===
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 +
{{Warning|The RAID 5 and RAID 6 modes of Btrfs are fatally flawed, and should not be used for "anything but testing with throw-away data." See [https://btrfs.wiki.kernel.org/index.php/RAID56 the Btrfs page on RAID5 and RAID6] for status updates.}}
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 +
Multiple devices can be used to create a RAID. Supported RAID levels include RAID 0, RAID 1, RAID 10, RAID 5 and RAID 6. The RAID levels can be configured separately for data and metadata using the {{ic|-d}} and {{ic|-m}} options respectively. By default the data is striped ({{ic|raid0}}) and the metadata is mirrored ({{ic|raid1}}). See [https://btrfs.wiki.kernel.org/index.php/Using_Btrfs_with_Multiple_Devices Using Btrfs with Multiple Devices] for more information about how to create a Btrfs RAID volume.
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 +
# mkfs.btrfs -d raid0 -m raid1 /dev/''part1'' /dev/''part2'' ...
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 +
You can create a [[W:JBOD|JBOD configuration]], where disks are seen as one filesystem, but files are not duplicated, using {{ic|-d single -m raid1}}.
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 +
You must include either the {{ic|udev}} hook or the {{ic|btrfs}} hook in {{ic|/etc/mkinitcpio.conf}} in order to use multiple Btrfs devices in a pool. See the [[Mkinitcpio#Common hooks]] article for more information.
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 +
{{Note|
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* It is possible to add devices to a multiple-device filesystem later on. See the [https://btrfs.wiki.kernel.org/index.php/Using_Btrfs_with_Multiple_Devices Btrfs wiki article] for more information.
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* Devices can be of different sizes. However, if one drive in a RAID configuration is bigger than the others, this extra space will not be used.
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* Some [[boot loader]]s such as [[Syslinux]] do not support multi-device file systems.
 
}}
 
}}
  
{{Warning|
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See [[#RAID]] for advice on maintenance specific to multi-device Btrfs file systems.
* As of kernel 3.17.x, using snapshots will cause [http://www.mail-archive.com/linux-btrfs@vger.kernel.org/msg38039.html random corruption] and you can't delete them without recreating the filesystem.
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== Configuring the file system ==
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 +
=== Copy-on-Write (CoW) ===
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By default, Btrfs uses [[Wikipedia:copy-on-write|copy-on-write]] for all files all the time. See the [https://btrfs.wiki.kernel.org/index.php/SysadminGuide#Copy_on_Write_.28CoW.29 Btrfs Sysadmin Guide section] for implementation details, as well as advantages and disadvantages.
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 +
==== Disabling CoW ====
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To disable copy-on-write for newly created files in a mounted subvolume, use the {{ic|nodatacow}} mount option. This will only affect newly created files. Copy-on-write will still happen for existing files. The {{ic|nodatacow}} option also disables compression. See {{man|5|btrfs}} for details.
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 +
{{Note|From {{man|5|btrfs|MOUNT OPTIONS}}: "within a single file system, it is not possible to mount some subvolumes with {{ic|nodatacow}} and others with {{ic|datacow}}. The mount option of the first mounted subvolume applies to any other subvolumes."}}
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 +
To disable copy-on-write for single files/directories do:
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 +
$ chattr +C ''/dir/file''
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 +
This will disable copy-on-write for those operation in which there is only one reference to the file. If there is more than one reference (e.g. through {{ic|1=cp --reflink=always}} or because of a filesystem snapshot), copy-on-write still occurs.
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 +
{{Note|From chattr man page: "For btrfs, the 'C' flag should be set on new or empty files. If it is set on a file which already has data blocks, it is undefined when the blocks assigned to the file will be fully stable. If the 'C' flag is set on a directory, it will have no effect on the directory, but new files created in that directory will have the No_COW attribute."}}
  
* It doesn't directly affect the normal filesystem, only the snapshot.
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{{Tip|In accordance with the note above, you can use the following trick to disable copy-on-write on existing files in a directory:
  
* Running {{ic|btrfs check --repair}} might render the filesystem unmountable.
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$ mv ''/path/to/dir'' ''/path/to/dir''_old
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$ mkdir ''/path/to/dir''
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$ chattr +C ''/path/to/dir''
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$ cp -a ''/path/to/dir''_old/* ''/path/to/dir''
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$ rm -rf ''/path/to/dir''_old
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 +
Make sure that the data are not used during this process. Also note that {{ic|mv}} or {{ic|cp --reflink}} as described below will not work.
 
}}
 
}}
  
== Installation ==
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==== Creating lightweight copies ====
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 +
By default, when copying files on a Btrfs filesystem with {{ic|cp}}, actual copies are created. To create a lightweight copy referencing to the original data, use the ''reflink'' option:
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 +
$ cp --reflink ''source'' ''dest''
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 +
See the man page on {{man|1|cp}} for more details on the {{ic|--reflink}} flag.
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 +
=== Compression ===
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Btrfs supports transparent and automatic compression. This not only reduces the size of files, but can also [http://www.phoronix.com/scan.php?page=article&item=btrfs_compress_2635&num=1 improve performance], in some specific use cases (e.g. single thread with heavy file I/O), while obviously harming performance in other cases (e.g. multithreaded and/or cpu intensive tasks with large file I/O). Better performance is generally achieved with the fastest compress algorithms, ''zstd'' and ''lzo'', and some [https://www.phoronix.com/scan.php?page=article&item=btrfs-zstd-compress benchmarks] provide detailed comparisons.
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 +
The {{ic|1=compress=''alg''}} mount option enables automatically considering every file for compression, where {{ic|''alg''}} is either {{ic|zlib}}, {{ic|lzo}}, {{ic|zstd}}, or {{ic|no}} (for no compression). Using this option, btrfs will check if compressing the first portion of the data shrinks it.  If it does, the entire write to that file will be compressed.  If it does not, none of it is compressed.  With this option, if the first portion of the write does not shrink, no compression will be applied to the write even if the rest of the data would shrink tremendously. [https://btrfs.wiki.kernel.org/index.php/Compression#What_happens_to_incompressible_files.3F]  This is done to prevent making the disk wait to start writing until all of the data to be written is fully given to btrfs and compressed.
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 +
The {{ic|1=compress-force=''alg''}} mount option can be used instead, which makes btrfs skip checking if compression shrinks the first portion, and enables automatic compression for every file.  In a worst case scenario, this can cause (slightly) more space to be used, and CPU usage for no purpose.
  
Btrfs is included in the default kernel and its tools ({{Pkg|btrfs-progs}}) are available in the official repositories. [[GRUB]], [[mkinitcpio]], and [[Syslinux]] have support for Btrfs and require no additional configuration.
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Only files created or modified after the mount option is added will be compressed.
  
=== Additional packages ===
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To apply compression to existing files, use the {{ic|btrfs filesystem defragment -c''alg''}} command, where {{ic|''alg''}} is either {{ic|zlib}}, {{ic|lzo}} or {{ic|zstd}}. For example, in order to re-compress the whole file system with {{pkg|zstd}}, run the following command:
  
* {{Pkg|btrfs-progs}} includes ''btrfsck'', a tool that can fix errors on Btrfs filesystems.
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# btrfs filesystem defragment -r -v -czstd /
* {{AUR|mkinitcpio-btrfs}} enables roll-back abilities.
 
* {{AUR|btrfs-progs-git}} for nightly
 
  
{{Tip|See [https://btrfs.wiki.kernel.org/index.php/Getting_started Btrfs Wiki Getting Started] for suggestions regarding running Btrfs effectively.}}
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To enable compression when installing Arch to an empty Btrfs partition, use the {{ic|compress}} option when [[mounting]] the file system: {{ic|1=mount -o compress=zstd /dev/sd''xY'' /mnt/}}. During configuration, add {{ic|1=compress=zstd}} to the mount options of the root file system in [[fstab]].
  
== General administration of Btrfs ==
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{{Tip|Compression can also be enabled per-file without using the {{ic|compress}} mount option; to do so apply {{ic|chattr +c}} to the file. When applied to directories, it will cause new files to be automatically compressed as they come.}}
  
=== Creating a new file system ===
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{{Warning|
 +
* Systems using older kernels or {{pkg|btrfs-progs}} without {{ic|zstd}} support may be unable to read or repair your filesystem if you use this option.
 +
* [[GRUB]] introduced ''zstd'' support in 2.04.  Make sure you have actually upgraded the bootloader installed in your MBR/ESP since then, by running {{ic|grub-install}} with the appropriate options for your BIOS/UEFI setup, since that is not done automatically. See {{Bug|63235}}.
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* [[rEFInd]] before version 0.11.4 lacks support for ''zstd'', either switch to {{AUR|refind-efi-git}}, use a separate boot partition without ''zstd'', or reset compression of boot files to something supported using for example the command: {{bc|$ btrfs filesystem defragment -v -clzo /boot/*}}
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}}
  
A Btrfs file system can either be newly created or have one converted.
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==== View compression types and ratios ====
  
To format a partition do:
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{{pkg|compsize}} takes a list of files (or an entire btrfs filesystem) and measures compression types used and effective compression ratios.  Uncompressed size may not match the number given by other programs such as {{ic|du}}, because every extent is counted once, even if it is reflinked several times, and even if part of it is no longer used anywhere but has not been garbage collected.  The {{ic|-x}} option keeps it on a single filesystem, which is useful in situations like {{ic|compsize -x /}} to avoid it from attempting to look in non-btrfs subdirectories and fail the entire run.
  
# mkfs.btrfs -L ''mylabel'' /dev/''partition''
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=== Subvolumes ===
  
{{Note|1=As of [https://git.kernel.org/cgit/linux/kernel/git/mason/btrfs-progs.git/commit/?id=c652e4efb8e2dd76ef1627d8cd649c6af5905902 this] commit (November 2013), Btrfs default blocksize is 16KB.}}
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"A btrfs subvolume is not a block device (and cannot be treated as one) instead, a btrfs subvolume can be thought of as a POSIX file namespace. This namespace can be accessed via the top-level subvolume of the filesystem, or it can be mounted in its own right." [https://btrfs.wiki.kernel.org/index.php/SysadminGuide#Subvolumes]
  
To use a larger blocksize for data/meta data, specify a value for the leafsize via the {{ic|-l}} switch as shown in this example using 16KB blocks:
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Each Btrfs file system has a top-level subvolume with ID 5. It can be mounted as {{ic|/}} (by default), or another subvolume can be [[#Mounting subvolumes|mounted]] instead. Subvolumes can be moved around in the filesystem and are rather identified by their id than their path.
  
# mkfs.btrfs -L ''mylabel'' -l 16k /dev/''partition''
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See the following links for more details:
 +
* [https://btrfs.wiki.kernel.org/index.php/SysadminGuide#Subvolumes Btrfs Wiki SysadminGuide#Subvolumes]
 +
* [https://btrfs.wiki.kernel.org/index.php/Getting_started#Basic_Filesystem_Commands Btrfs Wiki Getting started#Basic Filesystem Commands]
 +
* [https://btrfs.wiki.kernel.org/index.php/Trees Btrfs Wiki Trees]
  
Multiple devices can be entered to create a RAID. Supported RAID levels include RAID 0, RAID 1, RAID 10, RAID 5 and RAID 6. By default the metadata is mirrored and data is striped. See [https://btrfs.wiki.kernel.org/index.php/Using_Btrfs_with_Multiple_Devices Using Btrfs with Multiple Devices] for more information about how to create a Btrfs RAID volume.
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==== Creating a subvolume ====
  
# mkfs.btrfs [''options''] /dev/''part1'' /dev/''part2''
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To create a subvolume:
  
=== Convert from Ext3/4 ===
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# btrfs subvolume create ''/path/to/subvolume''
  
Boot from an install CD, then convert by doing:
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==== Listing subvolumes ====
  
# btrfs-convert /dev/''partition''
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To see a list of current subvolumes and their ids under {{ic|''path''}}:
  
Mount the partion and test the conversion by checking the files. Be sure to change the {{ic|/etc/fstab}} to reflect the change ('''type''' to {{ic|btrfs}} and '''fs_passno''' [the last field] to {{ic|0}} as Btrfs does not do a file system check on boot). Also note that the UUID of the partition will have changed, so update fstab accordingly when using UUIDs. {{ic|chroot}} into the system and rebuild the GRUB menu list (see [[Install from Existing Linux]] and [[GRUB]] articles). If converting a root filesystem, while still chrooted run {{ic|mkinitcpio -p linux}} to regenerate the initramfs or the system will not successfully boot.
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  # btrfs subvolume list -p ''path''
  
To complete, delete the saved image, delete the sub-volume that image is on, and finally [[#Balance|balance]] the file system to reclaim the space.
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==== Deleting a subvolume ====
  
# rm /ext2_saved/*
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To delete a subvolume:
# btrfs subvolume delete /ext2_saved
 
  
=== Mount options ===
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# btrfs subvolume delete ''/path/to/subvolume''
  
{{Warning|Specific mount options can disable safety features and increase the risk of complete file system corruption.}}
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Since Linux 4.18, one can also delete a subvolume like a regular directory ({{ic|<nowiki>rm -r</nowiki>}}, {{ic|<nowiki>rmdir</nowiki>}}).
  
See [https://btrfs.wiki.kernel.org/index.php/Mount_options Btrfs Wiki Mount options] and [https://btrfs.wiki.kernel.org/index.php/Gotchas Btrfs Wiki Gotchas] for more information.
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==== Mounting subvolumes ====
  
In addition to configurations that can be made during or after file system creation, the various mount options for Btrfs can drastically change its performance characteristics.
+
Subvolumes can be mounted like file system partitions using the {{ic|1=subvol=''/path/to/subvolume''}} or {{ic|1=subvolid=''objectid''}} mount flags. For example, you could have a subvolume named {{ic|subvol_root}} and mount it as {{ic|/}}.  One can mimic traditional file system partitions by creating various subvolumes under the top level of the file system and then mounting them at the appropriate mount points. Thus one can easily restore a file system (or part of it) to a previous state using [[#Snapshots]].
  
As this is a file system that is still in active development. Changes and regressions should be expected. See links in the [[#See also]] section for some benchmarks.
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{{Tip|1= Changing subvolume layouts is made simpler by not using the toplevel subvolume (ID=5) as {{ic|/}} (which is done by default). Instead, consider creating a subvolume to house your actual data and mounting it as {{ic|/}}.}}
  
==== Examples ====
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{{Note|From {{man|5|btrfs|MOUNT OPTIONS}}: "Most mount options apply to the '''whole filesystem''', and only the options for the first subvolume to be mounted will take effect. This is due to lack of implementation and may change in the future.". See the [https://btrfs.wiki.kernel.org/index.php/FAQ#Can_I_mount_subvolumes_with_different_mount_options.3F Btrfs Wiki FAQ] for which mount options can be used per subvolume.}}
  
* '''Linux 3.15'''
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See [[Snapper#Suggested filesystem layout]], [https://btrfs.wiki.kernel.org/index.php/SysadminGuide#Managing_Snapshots Btrfs SysadminGuide#Managing Snapshots], and [https://btrfs.wiki.kernel.org/index.php/SysadminGuide#Layout Btrfs SysadminGuide#Layout] for example file system layouts using subvolumes.
** Btrfs on a SSD for system installation and an emphasis on maximizing performance.
 
*:{{bc|1=noatime,discard,ssd,autodefrag,compress=lzo,space_cache}}
 
** Btrfs on a HDD for archival purposes with an emphasis on maximizing space.
 
*: {{bc|1=noatime,autodefrag,compress-force=lzo,space_cache}}
 
  
=== Displaying used/free space ===
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See {{man|5|btrfs}} for a full list of btrfs-specific mount options.
  
General linux userspace tools such as {{ic|/usr/bin/df}} will inaccurately report free space on a Btrfs partition since it does not take into account space allocated for and used by the metadata. It is recommended to use {{ic|/usr/bin/btrfs}} to query a btrfs partition. Below is an illustration of this effect, first querying using {{ic|df -h}}, and then using {{ic|btrfs filesystem df}}:
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==== Mounting subvolume as root ====
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To use a subvolume as the root mountpoint specify the subvolume via a [[Kernel parameters#Configuration|kernel parameter]] using {{ic|1=rootflags=subvol=''/path/to/subvolume''}}. Edit the root mountpoint in {{ic|/etc/fstab}} and specify the mount option {{ic|1=subvol=}}. Alternatively the subvolume can be specified with its id, {{ic|1=rootflags=subvolid=''objectid''}} as kernel parameter and {{ic|1=subvolid=''objectid''}} as mount option in {{ic|/etc/fstab}}.
  
{{hc|$ df -h /|
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==== Changing the default sub-volume ====
Filesystem      Size  Used Avail Use% Mounted on
 
/dev/sda3      119G  3.0G  116G  3% /
 
}}
 
  
{{hc|$ btrfs filesystem df /|2=
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The default sub-volume is mounted if no {{ic|1=subvol=}} mount option is provided. To change the default subvolume, do:
Data: total=3.01GB, used=2.73GB
 
System: total=4.00MB, used=16.00KB
 
Metadata: total=1.01GB, used=181.83MB
 
}}
 
  
Notice that {{ic|df -h}} reports 3.0GB used but {{ic|btrfs filesystem df}} reports 2.73GB for the data. This is due to the way Btrfs allocates space into the pool. The true disk usage is the sum of all three 'used' values which is inferior to 3.0GB as reported by {{ic|df -h}}.
+
# btrfs subvolume set-default ''subvolume-id'' /
  
{{Note|1=If you see an entry of type {{ic|unknown}} in the output of {{ic|btrfs filesystem df}} at kernel >= 3.15, this is a display bug. As of [http://thread.gmane.org/gmane.comp.file-systems.btrfs/34419 this patch], the entry means GlobalReserve, which is kind of a buffer for changes not yet flushed. This entry is displayed as {{ic|unknown, single}} in RAID setups and is not possible to re-balance.}}
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where ''subvolume-id'' can be found by [[#Listing subvolumes|listing]].
  
Another useful command to show a less verbose readout of used space is {{ic|btrfs filesystem show}}:
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{{Note|1=After changing the default subvolume on a system with [[GRUB]], you should run {{ic|grub-install}} again to notify the bootloader of the changes. See [https://bbs.archlinux.org/viewtopic.php?pid=1615373 this forum thread].}}
  
{{hc|# btrfs filesystem show /dev/sda3|
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Changing the default subvolume with {{ic|btrfs subvolume set-default}} will make the top level of the filesystem inaccessible, except by use of the {{ic|1=subvol=/}} or {{ic|1=subvolid=5}} mount options [https://btrfs.wiki.kernel.org/index.php/SysadminGuide].
failed to open /dev/sr0: No medium found
 
Label: 'arch64'  uuid: 02ad2ea2-be12-2233-8765-9e0a48e9303a
 
Total devices 1 FS bytes used 2.91GB
 
devid    1 size 118.95GB used 4.02GB path /dev/sda2
 
  
Btrfs v0.20-rc1-358-g194aa4a-dirty
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=== Quota ===
}}
 
  
== Limitations ==
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{{warning|Qgroup is not stable yet and combining quota with (too many) snapshots of subvolumes can cause performance problems, for example when deleting snapshots. Plus there are several more [https://btrfs.wiki.kernel.org/index.php/Quota_support#Known_issues known issues].}}
  
A few limitations should be known before trying.
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Quota support in Btrfs is implemented at a subvolume level by the use of quota groups or qgroup: Each subvolume is assigned a quota groups in the form of ''0/subvolume_id'' by default. However it is possible to create a quota group using any number if desired.
  
=== Encryption ===
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To use qgroups you need to enable quota first using
  
Btrfs has no built-in encryption support (this may come in future); users can encrypt the partition before running {{ic|mkfs.btrfs}}. See [[dm-crypt]].
+
# btrfs quota enable ''path''
  
Existing Btrfs file system, can use something like [[EncFS]] or [[TrueCrypt]], though perhaps without some of Btrfs' features.
+
From this point onwards newly created subvolumes will be controlled by those groups.
 +
In order to retrospectively enable them for already existing subvolumes, enable quota normally, then create a qgroup (quota group) for each of those subvolume using their ''subvolume_id'' and rescan them:
  
=== Swap file ===
+
# btrfs subvolume list ''path'' | cut -d' ' -f2 | xargs -I{} -n1 btrfs qgroup create 0/{} ''path''
 +
# btrfs quota rescan ''path''
  
Btrfs does not support [[Swap#Swap_file|swap files]]. This is due to swap files requiring a function that Btrfs doesn't have for possibility of file system corruption [https://btrfs.wiki.kernel.org/index.php/FAQ#Does_btrfs_support_swap_files.3F]. A swap file can be mounted on a loop device with poorer performance but will not be able to hibernate. Install the package {{Pkg|systemd-swap}} from the [[official repositories]] to automate this.
+
Quota groups in Btrfs form a tree hierarchy, whereby qgroups are attached to subvolumes. The size limits are set per qgroup and apply when any limit is reached in tree that contains a given subvolume.
  
=== Linux-rt kernel ===
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Limits on quota groups can be applied either to the total data usage, un-shared data usage, compressed data usage or both. File copy and file deletion may both affect limits since the unshared limit of another qgroup can change if the original volume's files are deleted and only one copy is remaining. For example a fresh snapshot shares almost all the blocks with the original subvolume, new writes to either subvolume will raise towards the exclusive limit, deletions of common data in one volume raises towards the exclusive limit in the other one.
  
As of version 3.14.12_rt9 the [[Kernel#-rt|linux-rt kernel]] will not boot with the Btrfs file system due slow development of the patchset. If you require the rt patchset please do your research on compatibility.
+
To apply a limit to a qgroup, use the command {{ic|btrfs qgroup limit}}. Depending on your usage either use a total limit, unshared limit ({{ic|-e}}) or compressed limit ({{ic|-c}}).
 +
To show usage and limits for a given path within a filesystem use
  
== Features ==
+
# btrfs qgroup show -reF ''path''
  
Various features are available and can be adjusted.
+
=== Commit interval ===
  
=== Copy-On-Write (CoW) ===
+
The resolution at which data are written to the filesystem is dictated by Btrfs itself and by system-wide settings. Btrfs defaults to a 30 seconds checkpoint interval in which new data are committed to the filesystem. This can be changed by appending the {{ic|commit}} mount option in {{ic|/etc/fstab}} for the btrfs partition.
  
The resolution at which data are written to the filesystem is dictated by Btrfs itself and by system-wide settings. Btrfs defaults to a 30 seconds checkpoint interval in which new data are committed to the filesystem. This is tuneable using mount options (see below)
+
LABEL=arch64 / btrfs defaults,noatime,compress=lzo,commit=120 0 0
  
 
System-wide settings also affect commit intervals. They include the files under {{ic|/proc/sys/vm/*}} and are out-of-scope of this wiki article. The kernel documentation for them resides in {{ic|Documentation/sysctl/vm.txt}}.
 
System-wide settings also affect commit intervals. They include the files under {{ic|/proc/sys/vm/*}} and are out-of-scope of this wiki article. The kernel documentation for them resides in {{ic|Documentation/sysctl/vm.txt}}.
  
CoW comes with some advantages, but can negatively affect performance with large files that have small random writes. It is recommended to disable CoW for database files and virtual machine images.
+
=== SSD TRIM ===
  
One can disable CoW for the entire block device by mounting it with {{ic|nodatacow}} option. However, this will disable CoW for the entire file system.
+
A Btrfs filesystem is able to free unused blocks from an SSD drive supporting the TRIM command.
  
{{Note|{{ic|nodatacow}} will only affect newly created file. CoW may still happen for existing file.}}
+
More information about enabling and using TRIM can be found in [[Solid State Drives#TRIM]].
  
To disable CoW for single files/directories do:
+
== Usage ==
  
$ chattr +C ''/dir/file''
+
=== Swap file ===
  
{{Note|From chattr man page: For btrfs, the 'C' flag should be set on new or empty files. If it is set on a file which already has data blocks, it is undefined when the blocks assigned to the file will be fully stable. If the 'C' flag is set on a directory, it will have no effect on the directory, but new files created in that directory will have the No_COW attribute.}}
+
[[Swap file]]s in Btrfs are supported since Linux kernel 5.0.[https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=ed46ff3d423780fa5173b38a844bf0fdb210a2a7] The proper way to initialize a swap file is described in [[Swap file#Swap file creation]]. Configuring hibernation to a swap file is described in [[Power management/Suspend and hibernate#Hibernation into swap file on Btrfs]].
  
{{Tip|In accordance with the note above, you can use the following trick to disable CoW on existing files in a directory:
+
{{Note|For kernels version 5.0+, Btfrs has native swap file support with some limitations:
$ mv ''/path/to/dir'' ''/path/to/dir''_old
+
* The swap file cannot be on a snapshotted subvolume. The proper procedure is to create a new subvolume to place the swap file in.
$ mkdir ''/path/to/dir''
+
* It does not support swap files on file systems that span multiple devices. See [https://btrfs.wiki.kernel.org/index.php/FAQ#Does_btrfs_support_swap_files.3F Btrfs wiki: Does btrfs support swap files?] and [https://bbs.archlinux.org/viewtopic.php?pid&#61;1849371#p1849371 Arch forums discussion].
$ chattr +C ''/path/to/dir''
 
$ cp -a ''/path/to/dir''_old/* ''/path/to/dir''
 
$ rm -rf ''/path/to/dir''_old
 
Make sure that the data are not used during this process. Also note that {{ic|mv}} or {{ic|cp --reflink}} as described below will not work.
 
 
}}
 
}}
  
Likewise, to save space by forcing CoW when copying files use:
+
{{Warning|Linux kernels before version 5.0, including {{pkg|linux-lts}}, do not support swap files. Using a swap file with Btrfs and a kernel prior to 5.0 can lead to file system corruption.}}
 +
 
 +
=== Displaying used/free space ===
 +
 
 +
General linux userspace tools such as {{ic|df}} will inaccurately report free space on a Btrfs partition. It is recommended to use {{ic|btrfs filesystem usage}} to query Btrfs partitions. For example:
  
  $ cp --reflink ''source'' ''dest''
+
  # btrfs filesystem usage /
  
As {{ic|''dest''}} file is changed, only those blocks that are changed from source will be written to the disk. One might consider aliasing aliasing ''cp'' to {{ic|1=cp --reflink=auto}}.
+
{{Note|The {{ic|btrfs filesystem usage}} command does not currently work correctly with {{ic|RAID5/RAID6}} RAID levels.}}
  
=== Multi-device filesystem and RAID feature ===
+
See [https://btrfs.wiki.kernel.org/index.php/FAQ#How_much_free_space_do_I_have.3F] for more information.
  
See [https://btrfs.wiki.kernel.org/index.php/Using_Btrfs_with_Multiple_Devices Using Btrfs with Multiple Devices] for suggestions.
+
=== Defragmentation ===
  
==== Multi-device filesystem ====
+
Btrfs supports online defragmentation through the mount option {{ic|autodefrag}}, see {{man|5|btrfs|MOUNT OPTIONS}}. To manually defragment your root, use:
  
When creating a ''btrfs'' filesystem, one can pass many partitions or disk devices to ''mkfs.btrfs''. The filesystem will be created across these devices. One can '''"'''pool'''"''' this way, multiple partitions or devices to get a big ''btrfs'' filesystem.
+
  # btrfs filesystem defragment -r /
  
One can also add or remove device from an existing btrfs filesystem (caution is mandatory).
+
Using the above command without the {{ic|-r}} switch will result in only the metadata held by the subvolume containing the directory being defragmented. This allows for single file defragmentation by simply specifying the path.
  
==== RAID features ====
+
Defragmenting a file which has a COW copy (either a snapshot copy or one made with {{ic|cp --reflink}} or bcp) plus using the {{ic|-c}} switch with a compression algorithm may result in two unrelated files effectively increasing the disk usage.
  
{{Warning|Parity RAID (RAID 5 and RAID 6) are not currently complete, and have significant problems with recovery from the loss of a device. They should not be used for anything other than testing purposes.}}
+
=== RAID ===
  
When creating multi-device filesystem, one can also specify to use RAID0, RAID1, RAID10, RAID5 or RAID6 across the devices comprising the filesystem. RAID levels can be applied independently to data and meta data. By default, meta data is duplicated on single volumes or RAID1 on multi-disk sets.
+
Btrfs offers native "RAID" for [[#Multi-device file system]]s. Notable features which set btrfs RAID apart from [[mdadm]] are self-healing redundant arrays and online balancing. See [https://btrfs.wiki.kernel.org/index.php/Using_Btrfs_with_Multiple_Devices the Btrfs wiki page] for more information. The Btrfs sysadmin page also [https://btrfs.wiki.kernel.org/index.php/SysadminGuide#RAID_and_data_replication has a section] with some more technical background.
  
btrfs works in block-pairs for raid0, raid1, and raid10. This means:
+
{{Warning| Parity RAID (RAID 5/6) code has multiple serious data-loss bugs in it. See the Btrfs Wiki's [https://btrfs.wiki.kernel.org/index.php/RAID56 RAID5/6 page] and a bug report on [https://www.mail-archive.com/linux-btrfs@vger.kernel.org/msg55161.html linux-btrfs mailing list] for more detailed information.}}
  
raid0 - block-pair striped across 2 devices
+
=== Scrub ===
  
raid1 - block-pair written to 2 devices
+
The [https://btrfs.wiki.kernel.org/index.php/Glossary Btrfs Wiki Glossary] says that Btrfs scrub is "[a]n online filesystem checking tool. Reads all the data and metadata on the filesystem, and uses checksums and the duplicate copies from RAID storage to identify and repair any corrupt data."
  
For 2 disk sets, this matches raid levels as defined in md-raid (mdadm). For 3+ disk-sets, the result is entirely different than md-raid.  
+
{{Warning|A running scrub process will prevent the system from suspending, see [http://comments.gmane.org/gmane.comp.file-systems.btrfs/33106 this thread] for details.}}
  
For example:
+
==== Start manually ====
* Three 1TB disks in an md based raid1 yields a {{ic|/dev/md0}} with 1TB free space and the ability to safely lose 2 disks without losing data.
 
* Three 1TB disks in a Btrfs volume with data=raid1 will allow the storage of approximately 1.5TB of data before reporting full. Only 1 disk can safely be lost without losing data.
 
  
Btrfs uses a round-robin scheme to decide how block-pairs are spread among disks. As of Linux 3.0, a quasi-round-robin scheme is used which prefers larger disks when distributing block pairs. This allows raid0 and raid1 to take advantage of most (and sometimes all) space in a disk set made of multiple disks. For example, a set consisting of a 1TB disk and 2 500GB disks with data=raid1 will place a copy of every block on the 1TB disk and alternate (round-robin) placing blocks on each of the 500GB disks. Full space utilization will be made. A set made from a 1TB disk, a 750GB disk, and a 500GB disk will work the same, but the filesystem will report full with 250GB unusable on the 750GB disk. To always take advantage of the full space (even in the last example), use data=single. (data=single is akin to JBOD defined by some raid controllers) See [https://btrfs.wiki.kernel.org/index.php/FAQ#How_much_space_do_I_get_with_unequal_devices_in_RAID-1_mode.3F the BTRFS FAQ] for more info.
+
To start a (background) scrub on the filesystem which contains {{ic|/}}:
  
=== Sub-volumes ===
+
# btrfs scrub start /
  
See the following links for more details:
+
To check the status of a running scrub:
* [https://btrfs.wiki.kernel.org/index.php/SysadminGuide#Subvolumes Btrfs Wiki SysadminGuide#Subvolumes]
 
* [https://btrfs.wiki.kernel.org/index.php/Getting_started#Basic_Filesystem_Commands Btrfs Wiki Getting started#Basic Filessystem Commands]
 
* [https://btrfs.wiki.kernel.org/index.php/Trees Btrfs Wiki Trees]
 
  
==== Creating sub-volumes ====
+
# btrfs scrub status /
  
To create a sub-volume:
+
==== Start with a service or timer ====
  
# btrfs subvolume create ''subvolume-name''
+
The {{Pkg|btrfs-progs}} package brings the {{ic|btrfs-scrub@.timer}} unit for monthly scrubbing the specified mountpoint. [[Enable]] the timer with an escaped path, e.g. {{ic|btrfs-scrub@-.timer}} for {{ic|/}} and {{ic|btrfs-scrub@home.timer}} for {{ic|/home}}. You can use {{ic|systemd-escape -p ''/path/to/mountpoint''}} to escape the path, see {{man|1|systemd-escape}} for details.
  
==== Listing sub-volumes ====
+
You can also run the scrub by [[starting]] {{ic|btrfs-scrub@.service}} (with the same encoded path). The advantage of this over {{ic|# btrfs scrub}} is that the results of the scrub will be logged in the [[systemd journal]].
  
To see a list of current sub-volumes:
+
=== Balance ===
  
# btrfs subvolume list -p .
+
"A balance passes all data in the filesystem through the allocator again. It is primarily intended to rebalance the data in the filesystem across the devices when a device is added or removed. A balance will regenerate missing copies for the redundant RAID levels, if a device has failed." [https://btrfs.wiki.kernel.org/index.php/Glossary] See [https://btrfs.wiki.kernel.org/index.php/FAQ#What_does_.22balance.22_do.3F Upstream FAQ page].
  
==== Setting a default sub-volume ====
+
On a single-device filesystem a balance may be also useful for (temporarily) reducing the amount of allocated but unused (meta)data chunks. Sometimes this is needed for fixing [https://btrfs.wiki.kernel.org/index.php/FAQ#Help.21_Btrfs_claims_I.27m_out_of_space.2C_but_it_looks_like_I_should_have_lots_left.21 "filesystem full" issues].
  
{{Warning|Changing the default subvolume with btrfs subvolume default will make the top level of the filesystem inaccessible, except by use of the {{ic|1=subvolid=0}} mount option. Reference: [https://btrfs.wiki.kernel.org/index.php/SysadminGuide Btrfs Wiki Sysadmin Guide].}}
+
# btrfs balance start /
 +
# btrfs balance status /
  
The default sub-volume is mounted if no {{ic|1=subvol=}} mount option is provided.
+
=== Snapshots ===
  
# btrfs subvolume set-default ''subvolume-id'' /.
+
"A snapshot is simply a subvolume that shares its data (and metadata) with some other subvolume, using btrfs's COW capabilities." See [https://btrfs.wiki.kernel.org/index.php/SysadminGuide#Snapshots Btrfs Wiki SysadminGuide#Snapshots] for details.
  
'''Example:'''
+
To create a snapshot:
  
{{hc|# btrfs subvolume list .|
+
# btrfs subvolume snapshot ''source'' [''dest''/]''name''
ID 258 gen 9512 top level 5 path root_subvolume
 
ID 259 gen 9512 top level 258 path home
 
ID 260 gen 9512 top level 258 path var
 
ID 261 gen 9512 top level 258 path usr
 
}}
 
  
# btrfs subvolume set-default 258 .
+
To create a readonly snapshot add the {{ic|-r}} flag. To create writable version of a readonly snapshot, simply create a snapshot of it.
  
'''Reset:'''
+
{{Note|Snapshots are not recursive. Every nested subvolume will be an empty directory inside the snapshot.}}
  
# btrfs subvolume set-default 0 .
+
=== Send/receive ===
  
==== Snapshots ====
+
A subvolume can be sent to stdout or a file using the {{ic|send}} command. This is usually most useful when piped to a Btrfs {{ic|receive}} command. For example, to send a snapshot named {{ic|/root_backup}} (perhaps of a snapshot you made of {{ic|/}} earlier) to {{ic|/backup}} you would do the following:
  
See [https://btrfs.wiki.kernel.org/index.php/SysadminGuide#Snapshots Btrfs Wiki SysadminGuide#Snapshots] for details.
+
  # btrfs send /root_backup | btrfs receive /backup
  
To create a snapshot:
+
The snapshot that is sent ''must'' be readonly. The above command is useful for copying a subvolume to an external device (e.g. a USB disk mounted at {{ic|/backup}} above).
  
# btrfs subvolume snapshot ''source'' [''dest''/]''name''
+
You can also send only the difference between two snapshots. For example, if you have already sent a copy of {{ic|root_backup}} above and have made a new readonly snapshot on your system named {{ic|root_backup_new}}, then to send only the incremental difference to {{ic|/backup}} do:
  
Snapshots are not recursive. Every sub-volume inside sub-volume will be an empty directory inside the snapshot.
+
  # btrfs send -p /root_backup /root_backup_new | btrfs receive /backup
  
==== Installing with root on btrfs subvolume ====
+
Now a new subvolume named {{ic|root_backup_new}} will be present in {{ic|/backup}}.
  
{{Moveto||Does not fit into [[#Features]] section, maybe we should start new one...}}
+
See [https://btrfs.wiki.kernel.org/index.php/Incremental_Backup Btrfs Wiki's Incremental Backup page] on how to use this for incremental backups and for tools that automate the process.
  
{{Accuracy|Setting {{ic|rootvol}} as the default subvolume (see [[#Setting a default sub-volume]]) would be much simpler, for both installation and booting.}}
+
=== Deduplication ===
  
{{Warning|When using legacy BIOS booting, syslinux is not capable of booting from a btrfs subvolume with the following setup, one must always use GRUB. (It is possible with a different, more complicated btrfs subvolume setup, that is out of the scope of this section.)}}
+
Using copy-on-write, Btrfs is able to copy files or whole subvolumes without actually copying the data. However whenever a file is altered a new ''proper'' copy is created. Deduplication takes this a step further, by actively identifying blocks of data which share common sequences and combining them into an extent with the same copy-on-write semantics.
  
For increased flexibility, install the system into a dedicated sub-volume.
+
Tools dedicated to deduplicate a Btrfs formatted partition include {{Pkg|duperemove}}, {{aur|bedup}} and ''btrfs-dedup''. One may also want to merely deduplicate data on a file based level instead using e.g. {{pkg|rmlint}} or {{aur|jdupes}}. For an overview of available features of those programs and additional information have a look at the [https://btrfs.wiki.kernel.org/index.php/Deduplication#Batch upstream Wiki entry].
Set up a btrfs filesystem as to your liking and mount it to /mnt. The following mkfs command is given as an example.
 
  
# mkfs.btrfs /dev/sda1
+
Furthermore Btrfs developers are working on inband (also known as synchronous or inline) deduplication, meaning deduplication done when writing new data to the filesystem. Currently it is still an experiment which is developed out-of-tree. Users willing to test the new feature should read the [https://btrfs.wiki.kernel.org/index.php/User_notes_on_dedupe appropriate kernel wiki page].
# mount /dev/sda1 /mnt
 
  
Create a subvolume for root, where rootvol is an arbitrary name. If you wish, create additional subvolumes at this step, for example to accommodate /home.
+
== Known issues ==
  
# cd /mnt
+
A few limitations should be known before trying.
# btrfs subvolume create rootvol
 
# cd /
 
# umount /mnt
 
  
Mount the subvolume.
+
=== Encryption ===
  
# mount -o subvol=rootvol /dev/sda1 /mnt
+
Btrfs has no built-in encryption support, but this [https://lwn.net/Articles/700487/ may] come in the future. Users can encrypt the partition before running {{ic|mkfs.btrfs}}. See [[dm-crypt/Encrypting an entire system#Btrfs subvolumes with swap]].
  
From here continue the installation process from the pacstrap step, as per the [[Installation guide]] or [[Beginners' guide]].
+
Existing Btrfs file systems can use something like [[EncFS]] or [[TrueCrypt]], though perhaps without some of Btrfs' features.
  
Additional considerations:
+
=== TLP ===
* In /etc/fstab, the mount option subvol="subvolume-name" has to be specified, and the fsck setting in the last field has to be 0.
 
* In the kernel boot parameters, use: {{ic|1=rootflags=subvol=''subvolume-name''}}. It is still necessary to add the standard root parameter with {{ic| 1=root=/dev/sda1}}.
 
* It is advisable to add '''crc32c''' (or '''crc32c-intel''' for Intel machines) to the modules array in {{ic|/etc/mkinitcpio.conf}}
 
  
{{Note|Child sub-volumes are automatically mounted by Btrfs when the parent sub-volume is mounted.}}
+
Using TLP requires special precautions in order to avoid filesystem corruption. Refer to the [[TLP#Btrfs|according TLP section]] for more information.
  
=== Defragmentation ===
+
=== btrfs check issues ===
 +
The tool {{ic|btrfs check}} has known issues and should not be run without further reading, see section [[#btrfs check]].
  
Btrfs supports online defragmentation. To defragment the metadata of the root folder do:
+
== Tips and tricks ==
  
# btrfs filesystem defragment /
+
=== Partitionless Btrfs disk ===
  
This ''will not'' defragment the entire file system. For more information read [https://btrfs.wiki.kernel.org/index.php/Problem_FAQ#Defragmenting_a_directory_doesn.27t_work this page] on the btrfs wiki.
+
{{Warning|Most users do not want this type of setup and instead should install Btrfs on a regular partition. Furthermore, GRUB strongly discourages installation to a partitionless disk. Consider using {{ic|--alloc-start}} for mkfs.btrfs to give larger space to GRUB. }}
  
To defragment the entire file system verbosely:
+
Btrfs can occupy an entire data storage device, replacing the [[MBR]] or [[GPT]] partitioning schemes, using [[#Subvolumes|subvolumes]] to simulate partitions. However, using a partitionless setup is not required to simply [[#File system creation|create a Btrfs filesystem]] on an existing [[partition]] that was created using another method. There are some limitations to partitionless single disk setups:
  
# btrfs filesystem defragment -r -v /
+
* Cannot place other [[file systems]] on another partition on the same disk.
 +
* If using a Linux kernel version before 5.0, you cannot use [[Swap|swap area]] as Btrfs did not support [[Swap#Swap_file|swap files]] pre-5.0 and there is no place to create [[Swap#Swap_partition|swap partition]]. This also limits the use of hibernation/resume, which needs a swap area to store the hibernation image. This includes the current LTS version of Linux which is on 4.xx.
 +
* Cannot use [[UEFI]] to boot.
  
{{Note|The command above will defragment only file data. To defragment directory metadata for every directory in the file system, run this command: {{bc|# find / -xdev -type d -print -exec btrfs filesystem defragment '{}' \;}}}}
+
To overwrite the existing partition table with Btrfs, run the following command:
  
=== Compression ===
+
# mkfs.btrfs /dev/sd''X''
  
Btrfs supports transparent compression, which means every file on the partition is automatically compressed. This does not only reduce the size of those files, but also [http://www.phoronix.com/scan.php?page=article&item=btrfs_compress_2635&num=1 improves performance], in particular if using the [http://www.phoronix.com/scan.php?page=article&item=btrfs_lzo_2638&num=1 lzo algorithm], in some specific use cases (e.g. single tread with heavy file IO), while obviously harming performance on other cases (e.g. multithreaded and/or cpu intensive tasks with large file IO).
+
For example, use {{ic|/dev/sda}} rather than {{ic|/dev/sda1}}. The latter would format an existing partition instead of replacing the entire partitioning scheme. Because the root partition is Btrfs, make sure {{ic|btrfs}} is compiled into the kernel, or put {{ic|btrfs}} into [[mkinitcpio.conf#MODULES]] and activate.
  
Compression is enabled using the {{ic|1=compress=zlib}} or {{ic|1=compress=lzo}} mount options. Only files created or modified after the mount option is added will be compressed. However, it can be applied quite easily to existing files (e.g. after a conversion from ext3/4) using the {{ic|btrfs filesystem defragment -c''alg''}} command, where {{ic|''alg''}} is either {{ic|zlib}} or {{ic|lzo}}. In order to re-compress the whole file system with {{ic|lzo}}, run the following command:
+
Install the [[boot loader]] like you would for a data storage device with a [[Master Boot Record]]. See [[Syslinux#Manual install]] or [[GRUB/Tips and tricks#Install to partition or partitionless disk]]. If your kernel does not boot due to {{ic|Failed to mount /sysroot.}}, please add {{ic|1=GRUB_PRELOAD_MODULES="btrfs"}} in {{ic|/etc/default/grub}} and generate the grub configuration ([[GRUB#Generate the main configuration file]]).
  
# btrfs filesystem defragment -r -v -clzo /
+
=== Ext3/4 to Btrfs conversion ===
  
{{Tip|Compression can also be enabled per-file without using the {{ic|compress}} mount option; simply apply {{ic|chattr +c}} to the file. When applied to directories, it will cause new files to be automatically compressed as they come.}}
+
{{Warning|There are many reports on the btrfs mailing list about incomplete/corrupt/broken conversions. Make sure you have ''working'' backups of any data you cannot afford to lose. See [https://btrfs.wiki.kernel.org/index.php/Conversion_from_Ext3 Conversion from Ext3] on the btrfs wiki for more information.}}
  
When installing Arch to an empty Btrfs partition, set the {{ic|compress}} option after [[Beginners' guide#Prepare_the_storage_drive|preparing the storage drive]]. Simply switch to another terminal ({{ic|Ctrl+Alt+''number''}}), and run the following command:
+
Boot from an install CD, then convert by doing:
  
  # mount -o remount,compress=lzo /mnt/target
+
  # btrfs-convert /dev/''partition''
  
After the installation is finished, add {{ic|1=compress=lzo}} to the mount options of the root file system in [[fstab]].
+
Mount the partion and test the conversion by checking the files.  Be sure to change the {{ic|/etc/fstab}} to reflect the change ('''type''' to {{ic|btrfs}} and '''fs_passno''' [the last field] to {{ic|0}} as Btrfs does not do a file system check on boot). Also note that the UUID of the partition will have changed, so update fstab accordingly when using UUIDs. {{ic|chroot}} into the system and rebuild the GRUB menu list (see [[Install from existing Linux]] and [[GRUB]] articles). If converting a root filesystem, while still chrooted run {{ic|mkinitcpio -p linux}} to regenerate the initramfs or the system will not successfully boot. If you get stuck in grub with 'unknown filesystem' try reinstalling grub with {{ic|grub-install /dev/''partition''}} and regenerate the config as well {{ic|grub-mkconfig -o /boot/grub/grub.cfg}}.
  
=== Checkpoint interval ===
+
After confirming that there are no problems, complete the conversion by deleting the backup {{ic|ext2_saved}} sub-volume. Note that you cannot revert back to ext3/4 without it.
  
Starting with Linux 3.12, users are able to change the checkpoint interval from the default 30 s to any value by appending the {{ic|commit}} mount option in {{ic|/etc/fstab}} for the btrfs partition.
+
# btrfs subvolume delete /ext2_saved
  
LABEL=arch64 / btrfs defaults,noatime,ssd,compress=lzo,commit=120 0 0
+
Finally [[#Balance|balance]] the file system to reclaim the space.
  
=== Partitioning ===
+
Remember that some applications which were installed prior have to be adapted to Btrfs. Notably [[TLP#Btrfs]] needs special care to avoid filesystem corruption but other applications may profit from certain features as well.
  
Btrfs can occupy an entire data storage device and replace the [[MBR]] or [[GPT]] partitioning schemes. One can use [[Btrfs#Sub-volumes|subvolumes]] to simulate partitions. There are some limitations to this approach in single disk setups:
+
=== Checksum hardware acceleration ===
  
* Cannot use different [[File systems|file systems]] for different [[fstab|mount points]].
+
To verify if Btrfs checksum is hardware accelerated:
* Cannot use [[Swap|swap area]] as Btrfs does not support [[Swap#Swap_file|swap files]] and there is no place to create [[Swap#Swap_partition|swap partition]].
+
{{hc|$ dmesg {{!}} grep crc32c|2=
* Cannot use [[UEFI]] to boot.
+
Btrfs loaded, crc32c=crc32c-intel
 +
}}
  
To overwrite the existing partition table with Btrfs, run the following command:
+
If you see {{ic|1=crc32c=crc32c-generic}}, it is probably because your root partition is Btrfs, and you will have to compile {{ic|crc32c-intel}} into the kernel to make it work. Putting {{ic|crc32c-intel}} into [[mkinitcpio.conf]] does ''not'' work.
  
# mkfs.btrfs /dev/sd''X''
+
=== Corruption recovery ===
  
Do not specify {{ic|/dev/sda''X''}} or it will format an existing partition instead of replacing the entire partitioning scheme.
+
{{Warning|The tool {{ic|btrfs check}} has known issues, see section [[#btrfs check]]}}
  
Install the [[Bootloaders|boot loader]] in a like fashion to installing it for a data storage device with a [[MBR|Master Boot Record]]. For example:
+
''btrfs-check'' cannot be used on a mounted file system. To be able to use ''btrfs-check'' without booting from a live USB, add it to the initial ramdisk:
  
  # grub-install --recheck /dev/sd''X''
+
{{hc|/etc/mkinitcpio.conf|output=
 +
  BINARIES=("/usr/bin/btrfs")
 +
}}
  
for [[Grub#Install_to_440-byte_MBR_boot_code_region|GRUB]].
+
[[Regenerate the initramfs]].
  
{{Warning|Using the {{ic|btrfs filesystem set-default}} command to change the default sub-volume from anything other than the top level (ID 0) may break Grub. See [[Btrfs#Setting a default sub-volume]] to reset.}}
+
Then if there is a problem booting, the utility is available for repair.
  
=== Scrub ===
+
{{Note|If the fsck process has to invalidate the space cache (and/or other caches?) then it is normal for a subsequent boot to hang up for a while (it may give console messages about btrfs-transaction being hung). The system should recover from this after a while.}}
  
See [https://btrfs.wiki.kernel.org/index.php/Glossary Btrfs Wiki Glossary].
+
See the [https://btrfs.wiki.kernel.org/index.php/Btrfsck Btrfs Wiki page] for more information.
  
# btrfs scrub start /
+
=== Booting into snapshots ===
# btrfs scrub status /
 
  
{{Warning|The running scrub process will prevent the system from suspending, see [http://comments.gmane.org/gmane.comp.file-systems.btrfs/33106 this thread] for details.}}
+
In order to boot into a snapshot, the same procedure applies as for mounting a subvolume as your root parition, as given in section [[#Mounting subvolume as root|mounting a subvolume as your root partition]], because snapshots can be mounted like subvolumes.
 +
If using GRUB you can automatically populate your boot menu with btrfs snapshots when regenerating the configuration file with the help of {{Pkg|grub-btrfs}} or {{AUR|grub-btrfs-git}}.
  
If running the scrub as a systemd service, use {{ic|1=Type=forking}}. Alternatively, you can pass the "-B" flag to btrfs scrub start to run it in the foreground and use the default Type value.
+
=== Use Btrfs subvolumes with systemd-nspawn ===
  
=== Balance ===
+
See the [[Systemd-nspawn#Use Btrfs subvolume as container root]] and [[Systemd-nspawn#Use temporary Btrfs snapshot of container]] articles.
  
See [https://btrfs.wiki.kernel.org/index.php/FAQ#What_does_.22balance.22_do.3F Upstream FAQ page].
+
=== Rootfs boot problems ===
  
Since {{Pkg|btrfs-progs}}-3.12 ''balancing'' is a background process - see {{ic|man 8 btrfs}} for full description.
+
Similarly to the conversion from Ext3/4 tip, if you're booting a btrfs partition as / you may run into boot problems if your ramdisk does not include support for btrfs. The error would look like {{ic|1=mount: unknown filesystem}}. To work around this issue, edit {{ic|1=/etc/mkinitcpio.conf}}  
 +
{{hc|/etc/mkinitcpio.conf|output=
 +
MODULES=(fs_btrfs)
 +
}}
  
# btrfs balance start /
+
Then, regenerate the ramdisk using {{ic|1=mkinitcpio -g /boot/initramfs-linux.img}}. Please be careful though. It may be a good idea to backup the working ramdisk, get comfortable with your bootloader so you can pass a different ramdisk as boot arg if needed be, and read about [https://projects.archlinux.org/mkinitcpio.git/ mkinitcpio]in the wiki.
# btrfs balance status /
 
  
 
== Troubleshooting ==
 
== Troubleshooting ==
 +
 +
See the [https://btrfs.wiki.kernel.org/index.php/Problem_FAQ Btrfs Problem FAQ] for general troubleshooting.
  
 
=== GRUB ===
 
=== GRUB ===
Line 377: Line 436:
 
==== Partition offset ====
 
==== Partition offset ====
  
{{Note|1=The offset problem may happen when you try to embed {{ic|core.img}} into a partitioned disk. It means that [https://wiki.archlinux.org/index.php?title=Talk:Btrfs&diff=319474&oldid=292530 it is OK] to embed grub's {{ic|corg.img}} into a Btrfs pool on a partitionless disk (e.g. {{ic|/dev/sd''X''}}) directly.}}
+
The offset problem may happen when you try to embed {{ic|core.img}} into a partitioned disk. It means that [https://wiki.archlinux.org/index.php?title=Talk:Btrfs&diff=319474&oldid=292530 it is OK] to embed grub's {{ic|core.img}} into a Btrfs pool on a partitionless disk (e.g. {{ic|/dev/sd''X''}}) directly.
  
[[GRUB]] can boot Btrfs partitions however the module may be larger than other [[file systems]]. And the {{ic|core.img}} file made by {{ic|grub-install}} may not fit in the first 63 sectors (31.5KiB) of the drive between the MBR and the first partition. Up-to-date partitioning tools such as {{ic|fdisk}} and {{ic|gdisk}} avoid this issue by offsetting the first partition by roughly 1MiB or 2MiB.
+
[[GRUB]] can boot Btrfs partitions, however the module may be larger than other [[file systems]]. And the {{ic|core.img}} file made by {{ic|grub-install}} may not fit in the first 63 sectors (31.5KiB) of the drive between the MBR and the first partition. Up-to-date partitioning tools such as {{ic|fdisk}} and {{ic|gdisk}} avoid this issue by offsetting the first partition by roughly 1MiB or 2MiB.
  
 
==== Missing root ====
 
==== Missing root ====
 +
 +
{{Accuracy|Suggests editing a non-configuration file manually.|Talk:Btrfs#Should not suggest to edit files in /usr/share}}
  
 
Users experiencing the following: {{ic|1=error no such device: root}} when booting from a RAID style setup then edit /usr/share/grub/grub-mkconfig_lib and remove both quotes from the line {{ic|1=echo "  search --no-floppy --fs-uuid --set=root ${hints} ${fs_uuid}"}}. Regenerate the config for grub and the system should boot without an error.
 
Users experiencing the following: {{ic|1=error no such device: root}} when booting from a RAID style setup then edit /usr/share/grub/grub-mkconfig_lib and remove both quotes from the line {{ic|1=echo "  search --no-floppy --fs-uuid --set=root ${hints} ${fs_uuid}"}}. Regenerate the config for grub and the system should boot without an error.
 +
 +
=== BTRFS: open_ctree failed ===
 +
 +
As of November 2014 there seems to be a bug in [[systemd]] or [[mkinitcpio]] causing the following error on systems with multi-device Btrfs filesystem using the {{ic|btrfs}} hook in {{ic|mkinitcpio.conf}}:
 +
 +
{{bc|<nowiki>
 +
BTRFS: open_ctree failed
 +
mount: wrong fs type, bad option, bad superblock on /dev/sdb2, missing codepage or helper program, or other error
 +
 +
In some cases useful info is found in syslog - try dmesg|tail or so.
 +
 +
You are now being dropped into an emergency shell.
 +
</nowiki>}}
 +
 +
A workaround is to remove {{ic|btrfs}} from the {{ic|HOOKS}} array in {{ic|/etc/mkinitcpio.conf}} and instead add {{ic|btrfs}} to the {{ic|MODULES}} array. Then regenerate the initramfs with {{ic|mkinitcpio -p linux}} (adjust the preset if needed) and reboot.
 +
 +
You will get the same error if you try to mount a raid array without one of the devices. In that case you must add the {{ic|degraded}} mount option to {{ic|/etc/fstab}}. If your root resides on the array, you must also add {{ic|1=rootflags=degraded}} to your [[kernel parameters]].
 +
 +
As of August 2016, a potential workaround for this bug is to mount the array by a single drive only in {{ic|/etc/fstab}}, and allow btrfs to discover and append the other drives automatically. Group-based identifiers such as UUID and LABEL appear to contribute to the failure. For example, a two-device RAID1 array consisting of 'disk1' and disk2' will have a UUID allocated to it, but instead of using the UUID, use only {{ic|/dev/mapper/disk1}} in {{ic|/etc/fstab}}. For a more detailed explanation, see the following [https://blog.samcater.com/fix-for-btrfs-open_ctree-failed-when-running-root-fs-on-raid-1-or-raid10-arch-linux/ blog post].
 +
 +
Another possible workaround is to remove the {{ic|udev}} hook in [[mkinitcpio.conf]] and replace it with the {{ic|systemd}} hook. In this case, {{ic|btrfs}} should ''not'' be in the {{ic|HOOKS}} or {{ic|MODULES}} arrays.
 +
 +
See the [https://bbs.archlinux.org/viewtopic.php?id=189845 original forums thread] and {{Bug|42884}} for further information and discussion.
 +
 +
=== btrfs check ===
 +
 +
{{Warning|Since Btrfs is under heavy development, especially the {{ic|btrfs check}} command, it is highly recommended to create a '''backup''' and consult the [https://btrfs.wiki.kernel.org/index.php/Btrfsck Btrfsck documentation] before executing {{ic|btrfs check}} with the {{ic|--repair}} switch.}}
 +
 +
The ''[https://btrfs.wiki.kernel.org/index.php/Manpage/btrfs-check btrfs check]'' command can be used to check or repair an unmounted Btrfs filesystem. However, this repair tool is still immature and not able to repair certain filesystem errors even those that do not render the filesystem unmountable.
  
 
== See also ==
 
== See also ==
Line 389: Line 479:
 
* '''Official site'''
 
* '''Official site'''
 
** [https://btrfs.wiki.kernel.org/ Btrfs Wiki]
 
** [https://btrfs.wiki.kernel.org/ Btrfs Wiki]
** [https://btrfs.wiki.kernel.org/index.php/Glossary Btrfs Wiki Glossary]
 
* '''Official FAQs'''
 
** [https://btrfs.wiki.kernel.org/index.php/FAQ Btrfs Wiki FAQ]
 
** [https://btrfs.wiki.kernel.org/index.php/Problem_FAQ Btrfs Wiki Problem FAQ]
 
* '''Btrfs pull requests'''
 
** [http://lkml.indiana.edu/hypermail/linux/kernel/1401.3/03045.html 3.14]
 
** [http://lkml.indiana.edu/hypermail/linux/kernel/1311.1/03526.html 3.13]
 
** [http://lkml.indiana.edu/hypermail/linux/kernel/1309.1/02981.html 3.12]
 
** [http://lkml.indiana.edu/hypermail/linux/kernel/1305.1/01064.html 3.11]
 
 
* '''Performance related'''
 
* '''Performance related'''
 
** [http://superuser.com/questions/432188/should-i-put-my-multi-device-btrfs-filesystem-on-disk-partitions-or-raw-devices Btrfs on raw disks?]
 
** [http://superuser.com/questions/432188/should-i-put-my-multi-device-btrfs-filesystem-on-disk-partitions-or-raw-devices Btrfs on raw disks?]
** [http://comments.gmane.org/gmane.comp.file-systems.btrfs/19440 Varying leafsize and nodesize in Btrfs]
+
** [https://www.spinics.net/lists/linux-btrfs/msg18652.html Varying leafsize and nodesize in Btrfs]
 
** [http://comments.gmane.org/gmane.comp.file-systems.btrfs/15646 Btrfs support for efficient SSD operation (data blocks alignment)]
 
** [http://comments.gmane.org/gmane.comp.file-systems.btrfs/15646 Btrfs support for efficient SSD operation (data blocks alignment)]
 
** [https://btrfs.wiki.kernel.org/index.php/FAQ#Is_Btrfs_optimized_for_SSD.3F Is Btrfs optimized for SSDs?]
 
** [https://btrfs.wiki.kernel.org/index.php/FAQ#Is_Btrfs_optimized_for_SSD.3F Is Btrfs optimized for SSDs?]
 
** '''Phoronix mount option benchmarking'''
 
** '''Phoronix mount option benchmarking'''
 +
*** [http://www.phoronix.com/scan.php?page=article&item=btrfs-mount-linux49 Linux 4.9]
 
*** [http://www.phoronix.com/scan.php?page=article&item=linux_314_btrfs Linux 3.14]
 
*** [http://www.phoronix.com/scan.php?page=article&item=linux_314_btrfs Linux 3.14]
 
*** [http://www.phoronix.com/scan.php?page=article&item=linux_btrfs_311&num=1 Linux 3.11]
 
*** [http://www.phoronix.com/scan.php?page=article&item=linux_btrfs_311&num=1 Linux 3.11]

Latest revision as of 13:16, 16 January 2020

From Btrfs Wiki:

Btrfs is a modern copy on write (CoW) filesystem for Linux aimed at implementing advanced features while also focusing on fault tolerance, repair and easy administration. Jointly developed at multiple companies, Btrfs is licensed under the GPL and open for contribution from anyone.
Warning: Btrfs has some features that are unstable. See the Btrfs Wiki's Status, Is Btrfs stable? and Getting started for more detailed information. See the #Known issues section.

Preparation

For user space utilities install the btrfs-progs package, which is required for basic operations.

If you need to boot from a Btrfs file system (i.e., your kernel and initramfs reside on a Btrfs partition), check if your boot loader supports Btrfs.

File system creation

The following shows how to create a new Btrfs file system. To convert an ext3/4 partition to Btrfs, see #Ext3/4 to Btrfs conversion. To use a partitionless setup, see #Partitionless Btrfs disk.

See mkfs.btrfs(8) for more information.

File system on a single device

To create a Btrfs filesystem on partition /dev/partition:

# mkfs.btrfs -L mylabel /dev/partition

The Btrfs default blocksize is 16KB. To use a larger blocksize for data/metadata, specify a value for the nodesize via the -n switch as shown in this example using 16KB blocks:

# mkfs.btrfs -L mylabel -n 16k /dev/partition

Multi-device file system

Warning: The RAID 5 and RAID 6 modes of Btrfs are fatally flawed, and should not be used for "anything but testing with throw-away data." See the Btrfs page on RAID5 and RAID6 for status updates.

Multiple devices can be used to create a RAID. Supported RAID levels include RAID 0, RAID 1, RAID 10, RAID 5 and RAID 6. The RAID levels can be configured separately for data and metadata using the -d and -m options respectively. By default the data is striped (raid0) and the metadata is mirrored (raid1). See Using Btrfs with Multiple Devices for more information about how to create a Btrfs RAID volume.

# mkfs.btrfs -d raid0 -m raid1 /dev/part1 /dev/part2 ...

You can create a JBOD configuration, where disks are seen as one filesystem, but files are not duplicated, using -d single -m raid1.

You must include either the udev hook or the btrfs hook in /etc/mkinitcpio.conf in order to use multiple Btrfs devices in a pool. See the Mkinitcpio#Common hooks article for more information.

Note:
  • It is possible to add devices to a multiple-device filesystem later on. See the Btrfs wiki article for more information.
  • Devices can be of different sizes. However, if one drive in a RAID configuration is bigger than the others, this extra space will not be used.
  • Some boot loaders such as Syslinux do not support multi-device file systems.

See #RAID for advice on maintenance specific to multi-device Btrfs file systems.

Configuring the file system

Copy-on-Write (CoW)

By default, Btrfs uses copy-on-write for all files all the time. See the Btrfs Sysadmin Guide section for implementation details, as well as advantages and disadvantages.

Disabling CoW

To disable copy-on-write for newly created files in a mounted subvolume, use the nodatacow mount option. This will only affect newly created files. Copy-on-write will still happen for existing files. The nodatacow option also disables compression. See btrfs(5) for details.

Note: From btrfs(5): "within a single file system, it is not possible to mount some subvolumes with nodatacow and others with datacow. The mount option of the first mounted subvolume applies to any other subvolumes."

To disable copy-on-write for single files/directories do:

$ chattr +C /dir/file

This will disable copy-on-write for those operation in which there is only one reference to the file. If there is more than one reference (e.g. through cp --reflink=always or because of a filesystem snapshot), copy-on-write still occurs.

Note: From chattr man page: "For btrfs, the 'C' flag should be set on new or empty files. If it is set on a file which already has data blocks, it is undefined when the blocks assigned to the file will be fully stable. If the 'C' flag is set on a directory, it will have no effect on the directory, but new files created in that directory will have the No_COW attribute."
Tip: In accordance with the note above, you can use the following trick to disable copy-on-write on existing files in a directory:
$ mv /path/to/dir /path/to/dir_old
$ mkdir /path/to/dir
$ chattr +C /path/to/dir
$ cp -a /path/to/dir_old/* /path/to/dir
$ rm -rf /path/to/dir_old

Make sure that the data are not used during this process. Also note that mv or cp --reflink as described below will not work.

Creating lightweight copies

By default, when copying files on a Btrfs filesystem with cp, actual copies are created. To create a lightweight copy referencing to the original data, use the reflink option:

$ cp --reflink source dest 

See the man page on cp(1) for more details on the --reflink flag.

Compression

Btrfs supports transparent and automatic compression. This not only reduces the size of files, but can also improve performance, in some specific use cases (e.g. single thread with heavy file I/O), while obviously harming performance in other cases (e.g. multithreaded and/or cpu intensive tasks with large file I/O). Better performance is generally achieved with the fastest compress algorithms, zstd and lzo, and some benchmarks provide detailed comparisons.

The compress=alg mount option enables automatically considering every file for compression, where alg is either zlib, lzo, zstd, or no (for no compression). Using this option, btrfs will check if compressing the first portion of the data shrinks it. If it does, the entire write to that file will be compressed. If it does not, none of it is compressed. With this option, if the first portion of the write does not shrink, no compression will be applied to the write even if the rest of the data would shrink tremendously. [1] This is done to prevent making the disk wait to start writing until all of the data to be written is fully given to btrfs and compressed.

The compress-force=alg mount option can be used instead, which makes btrfs skip checking if compression shrinks the first portion, and enables automatic compression for every file. In a worst case scenario, this can cause (slightly) more space to be used, and CPU usage for no purpose.

Only files created or modified after the mount option is added will be compressed.

To apply compression to existing files, use the btrfs filesystem defragment -calg command, where alg is either zlib, lzo or zstd. For example, in order to re-compress the whole file system with zstd, run the following command:

# btrfs filesystem defragment -r -v -czstd /

To enable compression when installing Arch to an empty Btrfs partition, use the compress option when mounting the file system: mount -o compress=zstd /dev/sdxY /mnt/. During configuration, add compress=zstd to the mount options of the root file system in fstab.

Tip: Compression can also be enabled per-file without using the compress mount option; to do so apply chattr +c to the file. When applied to directories, it will cause new files to be automatically compressed as they come.
Warning:
  • Systems using older kernels or btrfs-progs without zstd support may be unable to read or repair your filesystem if you use this option.
  • GRUB introduced zstd support in 2.04. Make sure you have actually upgraded the bootloader installed in your MBR/ESP since then, by running grub-install with the appropriate options for your BIOS/UEFI setup, since that is not done automatically. See FS#63235.
  • rEFInd before version 0.11.4 lacks support for zstd, either switch to refind-efi-gitAUR, use a separate boot partition without zstd, or reset compression of boot files to something supported using for example the command:
    $ btrfs filesystem defragment -v -clzo /boot/*

View compression types and ratios

compsize takes a list of files (or an entire btrfs filesystem) and measures compression types used and effective compression ratios. Uncompressed size may not match the number given by other programs such as du, because every extent is counted once, even if it is reflinked several times, and even if part of it is no longer used anywhere but has not been garbage collected. The -x option keeps it on a single filesystem, which is useful in situations like compsize -x / to avoid it from attempting to look in non-btrfs subdirectories and fail the entire run.

Subvolumes

"A btrfs subvolume is not a block device (and cannot be treated as one) instead, a btrfs subvolume can be thought of as a POSIX file namespace. This namespace can be accessed via the top-level subvolume of the filesystem, or it can be mounted in its own right." [2]

Each Btrfs file system has a top-level subvolume with ID 5. It can be mounted as / (by default), or another subvolume can be mounted instead. Subvolumes can be moved around in the filesystem and are rather identified by their id than their path.

See the following links for more details:

Creating a subvolume

To create a subvolume:

# btrfs subvolume create /path/to/subvolume

Listing subvolumes

To see a list of current subvolumes and their ids under path:

# btrfs subvolume list -p path

Deleting a subvolume

To delete a subvolume:

# btrfs subvolume delete /path/to/subvolume

Since Linux 4.18, one can also delete a subvolume like a regular directory (rm -r, rmdir).

Mounting subvolumes

Subvolumes can be mounted like file system partitions using the subvol=/path/to/subvolume or subvolid=objectid mount flags. For example, you could have a subvolume named subvol_root and mount it as /. One can mimic traditional file system partitions by creating various subvolumes under the top level of the file system and then mounting them at the appropriate mount points. Thus one can easily restore a file system (or part of it) to a previous state using #Snapshots.

Tip: Changing subvolume layouts is made simpler by not using the toplevel subvolume (ID=5) as / (which is done by default). Instead, consider creating a subvolume to house your actual data and mounting it as /.
Note: From btrfs(5): "Most mount options apply to the whole filesystem, and only the options for the first subvolume to be mounted will take effect. This is due to lack of implementation and may change in the future.". See the Btrfs Wiki FAQ for which mount options can be used per subvolume.

See Snapper#Suggested filesystem layout, Btrfs SysadminGuide#Managing Snapshots, and Btrfs SysadminGuide#Layout for example file system layouts using subvolumes.

See btrfs(5) for a full list of btrfs-specific mount options.

Mounting subvolume as root

To use a subvolume as the root mountpoint specify the subvolume via a kernel parameter using rootflags=subvol=/path/to/subvolume. Edit the root mountpoint in /etc/fstab and specify the mount option subvol=. Alternatively the subvolume can be specified with its id, rootflags=subvolid=objectid as kernel parameter and subvolid=objectid as mount option in /etc/fstab.

Changing the default sub-volume

The default sub-volume is mounted if no subvol= mount option is provided. To change the default subvolume, do:

# btrfs subvolume set-default subvolume-id /

where subvolume-id can be found by listing.

Note: After changing the default subvolume on a system with GRUB, you should run grub-install again to notify the bootloader of the changes. See this forum thread.

Changing the default subvolume with btrfs subvolume set-default will make the top level of the filesystem inaccessible, except by use of the subvol=/ or subvolid=5 mount options [3].

Quota

Warning: Qgroup is not stable yet and combining quota with (too many) snapshots of subvolumes can cause performance problems, for example when deleting snapshots. Plus there are several more known issues.

Quota support in Btrfs is implemented at a subvolume level by the use of quota groups or qgroup: Each subvolume is assigned a quota groups in the form of 0/subvolume_id by default. However it is possible to create a quota group using any number if desired.

To use qgroups you need to enable quota first using

# btrfs quota enable path

From this point onwards newly created subvolumes will be controlled by those groups. In order to retrospectively enable them for already existing subvolumes, enable quota normally, then create a qgroup (quota group) for each of those subvolume using their subvolume_id and rescan them:

# btrfs subvolume list path | cut -d' ' -f2 | xargs -I{} -n1 btrfs qgroup create 0/{} path
# btrfs quota rescan path

Quota groups in Btrfs form a tree hierarchy, whereby qgroups are attached to subvolumes. The size limits are set per qgroup and apply when any limit is reached in tree that contains a given subvolume.

Limits on quota groups can be applied either to the total data usage, un-shared data usage, compressed data usage or both. File copy and file deletion may both affect limits since the unshared limit of another qgroup can change if the original volume's files are deleted and only one copy is remaining. For example a fresh snapshot shares almost all the blocks with the original subvolume, new writes to either subvolume will raise towards the exclusive limit, deletions of common data in one volume raises towards the exclusive limit in the other one.

To apply a limit to a qgroup, use the command btrfs qgroup limit. Depending on your usage either use a total limit, unshared limit (-e) or compressed limit (-c). To show usage and limits for a given path within a filesystem use

# btrfs qgroup show -reF path

Commit interval

The resolution at which data are written to the filesystem is dictated by Btrfs itself and by system-wide settings. Btrfs defaults to a 30 seconds checkpoint interval in which new data are committed to the filesystem. This can be changed by appending the commit mount option in /etc/fstab for the btrfs partition.

LABEL=arch64 / btrfs defaults,noatime,compress=lzo,commit=120 0 0

System-wide settings also affect commit intervals. They include the files under /proc/sys/vm/* and are out-of-scope of this wiki article. The kernel documentation for them resides in Documentation/sysctl/vm.txt.

SSD TRIM

A Btrfs filesystem is able to free unused blocks from an SSD drive supporting the TRIM command.

More information about enabling and using TRIM can be found in Solid State Drives#TRIM.

Usage

Swap file

Swap files in Btrfs are supported since Linux kernel 5.0.[4] The proper way to initialize a swap file is described in Swap file#Swap file creation. Configuring hibernation to a swap file is described in Power management/Suspend and hibernate#Hibernation into swap file on Btrfs.

Note: For kernels version 5.0+, Btfrs has native swap file support with some limitations:
Warning: Linux kernels before version 5.0, including linux-lts, do not support swap files. Using a swap file with Btrfs and a kernel prior to 5.0 can lead to file system corruption.

Displaying used/free space

General linux userspace tools such as df will inaccurately report free space on a Btrfs partition. It is recommended to use btrfs filesystem usage to query Btrfs partitions. For example:

# btrfs filesystem usage /
Note: The btrfs filesystem usage command does not currently work correctly with RAID5/RAID6 RAID levels.

See [5] for more information.

Defragmentation

Btrfs supports online defragmentation through the mount option autodefrag, see btrfs(5). To manually defragment your root, use:

# btrfs filesystem defragment -r /

Using the above command without the -r switch will result in only the metadata held by the subvolume containing the directory being defragmented. This allows for single file defragmentation by simply specifying the path.

Defragmenting a file which has a COW copy (either a snapshot copy or one made with cp --reflink or bcp) plus using the -c switch with a compression algorithm may result in two unrelated files effectively increasing the disk usage.

RAID

Btrfs offers native "RAID" for #Multi-device file systems. Notable features which set btrfs RAID apart from mdadm are self-healing redundant arrays and online balancing. See the Btrfs wiki page for more information. The Btrfs sysadmin page also has a section with some more technical background.

Warning: Parity RAID (RAID 5/6) code has multiple serious data-loss bugs in it. See the Btrfs Wiki's RAID5/6 page and a bug report on linux-btrfs mailing list for more detailed information.

Scrub

The Btrfs Wiki Glossary says that Btrfs scrub is "[a]n online filesystem checking tool. Reads all the data and metadata on the filesystem, and uses checksums and the duplicate copies from RAID storage to identify and repair any corrupt data."

Warning: A running scrub process will prevent the system from suspending, see this thread for details.

Start manually

To start a (background) scrub on the filesystem which contains /:

# btrfs scrub start /

To check the status of a running scrub:

# btrfs scrub status /

Start with a service or timer

The btrfs-progs package brings the btrfs-scrub@.timer unit for monthly scrubbing the specified mountpoint. Enable the timer with an escaped path, e.g. btrfs-scrub@-.timer for / and btrfs-scrub@home.timer for /home. You can use systemd-escape -p /path/to/mountpoint to escape the path, see systemd-escape(1) for details.

You can also run the scrub by starting btrfs-scrub@.service (with the same encoded path). The advantage of this over # btrfs scrub is that the results of the scrub will be logged in the systemd journal.

Balance

"A balance passes all data in the filesystem through the allocator again. It is primarily intended to rebalance the data in the filesystem across the devices when a device is added or removed. A balance will regenerate missing copies for the redundant RAID levels, if a device has failed." [6] See Upstream FAQ page.

On a single-device filesystem a balance may be also useful for (temporarily) reducing the amount of allocated but unused (meta)data chunks. Sometimes this is needed for fixing "filesystem full" issues.

# btrfs balance start /
# btrfs balance status /

Snapshots

"A snapshot is simply a subvolume that shares its data (and metadata) with some other subvolume, using btrfs's COW capabilities." See Btrfs Wiki SysadminGuide#Snapshots for details.

To create a snapshot:

# btrfs subvolume snapshot source [dest/]name

To create a readonly snapshot add the -r flag. To create writable version of a readonly snapshot, simply create a snapshot of it.

Note: Snapshots are not recursive. Every nested subvolume will be an empty directory inside the snapshot.

Send/receive

A subvolume can be sent to stdout or a file using the send command. This is usually most useful when piped to a Btrfs receive command. For example, to send a snapshot named /root_backup (perhaps of a snapshot you made of / earlier) to /backup you would do the following:

 # btrfs send /root_backup | btrfs receive /backup

The snapshot that is sent must be readonly. The above command is useful for copying a subvolume to an external device (e.g. a USB disk mounted at /backup above).

You can also send only the difference between two snapshots. For example, if you have already sent a copy of root_backup above and have made a new readonly snapshot on your system named root_backup_new, then to send only the incremental difference to /backup do:

 # btrfs send -p /root_backup /root_backup_new | btrfs receive /backup

Now a new subvolume named root_backup_new will be present in /backup.

See Btrfs Wiki's Incremental Backup page on how to use this for incremental backups and for tools that automate the process.

Deduplication

Using copy-on-write, Btrfs is able to copy files or whole subvolumes without actually copying the data. However whenever a file is altered a new proper copy is created. Deduplication takes this a step further, by actively identifying blocks of data which share common sequences and combining them into an extent with the same copy-on-write semantics.

Tools dedicated to deduplicate a Btrfs formatted partition include duperemove, bedupAUR and btrfs-dedup. One may also want to merely deduplicate data on a file based level instead using e.g. rmlint or jdupesAUR. For an overview of available features of those programs and additional information have a look at the upstream Wiki entry.

Furthermore Btrfs developers are working on inband (also known as synchronous or inline) deduplication, meaning deduplication done when writing new data to the filesystem. Currently it is still an experiment which is developed out-of-tree. Users willing to test the new feature should read the appropriate kernel wiki page.

Known issues

A few limitations should be known before trying.

Encryption

Btrfs has no built-in encryption support, but this may come in the future. Users can encrypt the partition before running mkfs.btrfs. See dm-crypt/Encrypting an entire system#Btrfs subvolumes with swap.

Existing Btrfs file systems can use something like EncFS or TrueCrypt, though perhaps without some of Btrfs' features.

TLP

Using TLP requires special precautions in order to avoid filesystem corruption. Refer to the according TLP section for more information.

btrfs check issues

The tool btrfs check has known issues and should not be run without further reading, see section #btrfs check.

Tips and tricks

Partitionless Btrfs disk

Warning: Most users do not want this type of setup and instead should install Btrfs on a regular partition. Furthermore, GRUB strongly discourages installation to a partitionless disk. Consider using --alloc-start for mkfs.btrfs to give larger space to GRUB.

Btrfs can occupy an entire data storage device, replacing the MBR or GPT partitioning schemes, using subvolumes to simulate partitions. However, using a partitionless setup is not required to simply create a Btrfs filesystem on an existing partition that was created using another method. There are some limitations to partitionless single disk setups:

  • Cannot place other file systems on another partition on the same disk.
  • If using a Linux kernel version before 5.0, you cannot use swap area as Btrfs did not support swap files pre-5.0 and there is no place to create swap partition. This also limits the use of hibernation/resume, which needs a swap area to store the hibernation image. This includes the current LTS version of Linux which is on 4.xx.
  • Cannot use UEFI to boot.

To overwrite the existing partition table with Btrfs, run the following command:

# mkfs.btrfs /dev/sdX

For example, use /dev/sda rather than /dev/sda1. The latter would format an existing partition instead of replacing the entire partitioning scheme. Because the root partition is Btrfs, make sure btrfs is compiled into the kernel, or put btrfs into mkinitcpio.conf#MODULES and activate.

Install the boot loader like you would for a data storage device with a Master Boot Record. See Syslinux#Manual install or GRUB/Tips and tricks#Install to partition or partitionless disk. If your kernel does not boot due to Failed to mount /sysroot., please add GRUB_PRELOAD_MODULES="btrfs" in /etc/default/grub and generate the grub configuration (GRUB#Generate the main configuration file).

Ext3/4 to Btrfs conversion

Warning: There are many reports on the btrfs mailing list about incomplete/corrupt/broken conversions. Make sure you have working backups of any data you cannot afford to lose. See Conversion from Ext3 on the btrfs wiki for more information.

Boot from an install CD, then convert by doing:

# btrfs-convert /dev/partition

Mount the partion and test the conversion by checking the files. Be sure to change the /etc/fstab to reflect the change (type to btrfs and fs_passno [the last field] to 0 as Btrfs does not do a file system check on boot). Also note that the UUID of the partition will have changed, so update fstab accordingly when using UUIDs. chroot into the system and rebuild the GRUB menu list (see Install from existing Linux and GRUB articles). If converting a root filesystem, while still chrooted run mkinitcpio -p linux to regenerate the initramfs or the system will not successfully boot. If you get stuck in grub with 'unknown filesystem' try reinstalling grub with grub-install /dev/partition and regenerate the config as well grub-mkconfig -o /boot/grub/grub.cfg.

After confirming that there are no problems, complete the conversion by deleting the backup ext2_saved sub-volume. Note that you cannot revert back to ext3/4 without it.

# btrfs subvolume delete /ext2_saved

Finally balance the file system to reclaim the space.

Remember that some applications which were installed prior have to be adapted to Btrfs. Notably TLP#Btrfs needs special care to avoid filesystem corruption but other applications may profit from certain features as well.

Checksum hardware acceleration

To verify if Btrfs checksum is hardware accelerated:

$ dmesg | grep crc32c
Btrfs loaded, crc32c=crc32c-intel

If you see crc32c=crc32c-generic, it is probably because your root partition is Btrfs, and you will have to compile crc32c-intel into the kernel to make it work. Putting crc32c-intel into mkinitcpio.conf does not work.

Corruption recovery

Warning: The tool btrfs check has known issues, see section #btrfs check

btrfs-check cannot be used on a mounted file system. To be able to use btrfs-check without booting from a live USB, add it to the initial ramdisk:

/etc/mkinitcpio.conf
BINARIES=("/usr/bin/btrfs")

Regenerate the initramfs.

Then if there is a problem booting, the utility is available for repair.

Note: If the fsck process has to invalidate the space cache (and/or other caches?) then it is normal for a subsequent boot to hang up for a while (it may give console messages about btrfs-transaction being hung). The system should recover from this after a while.

See the Btrfs Wiki page for more information.

Booting into snapshots

In order to boot into a snapshot, the same procedure applies as for mounting a subvolume as your root parition, as given in section mounting a subvolume as your root partition, because snapshots can be mounted like subvolumes. If using GRUB you can automatically populate your boot menu with btrfs snapshots when regenerating the configuration file with the help of grub-btrfs or grub-btrfs-gitAUR.

Use Btrfs subvolumes with systemd-nspawn

See the Systemd-nspawn#Use Btrfs subvolume as container root and Systemd-nspawn#Use temporary Btrfs snapshot of container articles.

Rootfs boot problems

Similarly to the conversion from Ext3/4 tip, if you're booting a btrfs partition as / you may run into boot problems if your ramdisk does not include support for btrfs. The error would look like mount: unknown filesystem. To work around this issue, edit /etc/mkinitcpio.conf

/etc/mkinitcpio.conf
MODULES=(fs_btrfs)

Then, regenerate the ramdisk using mkinitcpio -g /boot/initramfs-linux.img. Please be careful though. It may be a good idea to backup the working ramdisk, get comfortable with your bootloader so you can pass a different ramdisk as boot arg if needed be, and read about mkinitcpioin the wiki.

Troubleshooting

See the Btrfs Problem FAQ for general troubleshooting.

GRUB

Partition offset

The offset problem may happen when you try to embed core.img into a partitioned disk. It means that it is OK to embed grub's core.img into a Btrfs pool on a partitionless disk (e.g. /dev/sdX) directly.

GRUB can boot Btrfs partitions, however the module may be larger than other file systems. And the core.img file made by grub-install may not fit in the first 63 sectors (31.5KiB) of the drive between the MBR and the first partition. Up-to-date partitioning tools such as fdisk and gdisk avoid this issue by offsetting the first partition by roughly 1MiB or 2MiB.

Missing root

Tango-inaccurate.pngThe factual accuracy of this article or section is disputed.Tango-inaccurate.png

Reason: Suggests editing a non-configuration file manually. (Discuss in Talk:Btrfs#Should not suggest to edit files in /usr/share)

Users experiencing the following: error no such device: root when booting from a RAID style setup then edit /usr/share/grub/grub-mkconfig_lib and remove both quotes from the line echo " search --no-floppy --fs-uuid --set=root ${hints} ${fs_uuid}". Regenerate the config for grub and the system should boot without an error.

BTRFS: open_ctree failed

As of November 2014 there seems to be a bug in systemd or mkinitcpio causing the following error on systems with multi-device Btrfs filesystem using the btrfs hook in mkinitcpio.conf:

BTRFS: open_ctree failed
mount: wrong fs type, bad option, bad superblock on /dev/sdb2, missing codepage or helper program, or other error

In some cases useful info is found in syslog - try dmesg|tail or so.

You are now being dropped into an emergency shell.

A workaround is to remove btrfs from the HOOKS array in /etc/mkinitcpio.conf and instead add btrfs to the MODULES array. Then regenerate the initramfs with mkinitcpio -p linux (adjust the preset if needed) and reboot.

You will get the same error if you try to mount a raid array without one of the devices. In that case you must add the degraded mount option to /etc/fstab. If your root resides on the array, you must also add rootflags=degraded to your kernel parameters.

As of August 2016, a potential workaround for this bug is to mount the array by a single drive only in /etc/fstab, and allow btrfs to discover and append the other drives automatically. Group-based identifiers such as UUID and LABEL appear to contribute to the failure. For example, a two-device RAID1 array consisting of 'disk1' and disk2' will have a UUID allocated to it, but instead of using the UUID, use only /dev/mapper/disk1 in /etc/fstab. For a more detailed explanation, see the following blog post.

Another possible workaround is to remove the udev hook in mkinitcpio.conf and replace it with the systemd hook. In this case, btrfs should not be in the HOOKS or MODULES arrays.

See the original forums thread and FS#42884 for further information and discussion.

btrfs check

Warning: Since Btrfs is under heavy development, especially the btrfs check command, it is highly recommended to create a backup and consult the Btrfsck documentation before executing btrfs check with the --repair switch.

The btrfs check command can be used to check or repair an unmounted Btrfs filesystem. However, this repair tool is still immature and not able to repair certain filesystem errors even those that do not render the filesystem unmountable.

See also