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[[Category:File systems]]
 
[[Category:File systems]]
 
[[ja:Btrfs]]
 
[[ja:Btrfs]]
[[zh-CN:Btrfs]]
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[[ru:Btrfs]]
{{Article summary start}}
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[[zh-hans:Btrfs]]
{{Article summary text|Provides an overview and setup of Btrfs on Arch Linux.}}
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{{Related articles start}}
{{Article summary heading|Related}}
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{{Related|File systems}}
{{Article summary wiki|Installing on Btrfs root}}
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{{Related|Snapper}}
{{Article summary wiki|Btrfs - Tips and tricks}}
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{{Related|dm-crypt/Encrypting an entire system#Btrfs subvolumes with swap}}
{{Article summary end}}
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{{Related articles end}}
  
Btrfs is an abbreviation for ''B-tree file system'' and is also known as ''Butter FS'' or ''Better FS''.  Btrfs is a copy-on-write (COW) file system written from the ground up for Linux. It is aimed at implementing advanced features while focusing on fault tolerance, repair and easy administration. Jointly developed by Oracle, Red Hat, Fujitsu, Intel, SUSE and many others, Btrfs is licensed under the GPL and open for contribution from anyone.
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From [[Wikipedia:Btrfs]]:
  
== Installation ==
+
:Btrfs (B-tree file system, pronounced as "butter F S", "better F S", "b-tree F S", or simply by spelling it out) is a file system based on the copy-on-write (COW) principle, initially designed at Oracle Corporation for use in Linux. The development of Btrfs began in 2007, and by August 2014, the file system's on-disk format has been marked as stable.
  
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.
+
From [https://btrfs.wiki.kernel.org/index.php/Main_Page Btrfs Wiki]:
  
* On 2012-03-28, {{Pkg|btrfs-progs}} includes ''btrfsck'', a tool that can fix errors on Btrfs filesystems.
+
: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.
* {{AUR|mkinitcpio-btrfs}}: for roll-back abilities (currently unmaintained).
 
  
{{Note|It is recommended that users use the most up-to-date version of of userspace utils as possible. Consider using {{AUR|btrfs-progs-git}} from the AUR and remember to rebuild it periodically or consider pulling btrfs-progs from the testing repo at a minimum.}}
+
{{Warning| Btrfs has some features that are considered experimental. 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.
 +
}}
  
== General administration of BTRFS ==
+
== Preparation ==
  
=== Creating a new file system ===
+
The official kernels {{Pkg|linux}} and {{Pkg|linux-lts}} include support for Btrfs. If you want to boot from a Btrfs file system, check if your [[boot loader]] supports Btrfs.
  
A Btrfs file system can either be newly created or have one converted.
+
User space utilities are available by [[installing]] the {{Pkg|btrfs-progs}} package.
 +
 
 +
== 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]].
 +
 
 +
=== File system on a single device ===
  
 
To format a partition do:
 
To format a partition do:
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  # mkfs.btrfs -L ''mylabel'' /dev/''partition''
 
  # mkfs.btrfs -L ''mylabel'' /dev/''partition''
  
{{Note|1=As [https://git.kernel.org/cgit/linux/kernel/git/mason/btrfs-progs.git/commit/?id=c652e4efb8e2dd76ef1627d8cd649c6af5905902 this] commit (November 2013), Btrfs default blocksize is 16KB.}}
+
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''
 +
 
 +
=== Multi-device file system ===
 +
 
 +
{{Warning|
 +
* The RAID 5, RAID 6 mode of Btrfs is considered ''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.
 +
* Some [[boot loaders]] such as [[Syslinux]] do not support multi-device file systems.}}
 +
 
 +
Multiple devices can be entered 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 as well as the manpage for {{ic|mkfs.btrfs}}.
 +
 
 +
# mkfs.btrfs -d raid0 -m raid1 /dev/''part1'' /dev/''part2'' ...
 +
 
 +
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.
 +
 
 +
{{Note|If the disks in your multi-disk array have different sizes, this may not use the full capacity of all drives. In order to utilize the full capacity of all disks, use {{ic|-d single}} instead of {{ic|-d raid0 -m raid1}} (metadata mirrored, data not mirrored and not striped)}}
 +
 
 +
{{Note|Mounting such a filesystem may result in all but one of the according ''.device''-jobs getting stuck and systemd never finishing startup due to a [https://github.com/systemd/systemd/issues/1921 bug] in handling this type of filesystem.}}
 +
 
 +
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 [[Wikipedia:copy-on-write]] for all files all the time. See [https://btrfs.wiki.kernel.org/index.php/SysadminGuide#Copy_on_Write_.28CoW.29 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 {{ic|nodatacow}} mount option. This will only affect newly created files. Copy-on-write will still happen for existing files.
 +
 
 +
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 {{ic|1=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 {{ic|mv}} or {{ic|cp --reflink}} as described below will not work.
 +
}}
 +
 
 +
==== Forcing CoW ====
 +
 
 +
To force copy-on-write when copying files use:
 +
 
 +
$ cp --reflink ''source'' ''dest''
 +
 
 +
This would only be required if CoW was disabled for the file to be copied (as implemented above). See the man page on {{ic|cp}} for more details on the {{ic|--reflink}} flag.
 +
 
 +
=== Compression ===
 +
 
 +
Btrfs supports transparent compression, meaning every file on the partition is automatically compressed. This not only reduces the size of 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 thread with heavy file IO), while obviously harming performance on other cases (e.g. multithreaded and/or cpu intensive tasks with large file IO).
 +
 
 +
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 {{pkg|lzo}}, run the following command:
 +
 
 +
# btrfs filesystem defragment -r -v -clzo /
 +
 
 +
{{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.}}
 +
 
 +
When installing Arch to an empty Btrfs partition, use the {{ic|compress}} option when [[mounting]] the file system: {{ic|1=mount -o compress=lzo /dev/sd''xY'' /mnt/}}. During configuration, add {{ic|1=compress=lzo}} to the mount options of the root file system in [[fstab]].
 +
 
 +
=== 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." [https://btrfs.wiki.kernel.org/index.php/SysadminGuide#Subvolumes]
 +
 
 +
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.
 +
 
 +
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]
 +
 
 +
==== Creating a subvolume ====
 +
 
 +
To create a subvolume:
 +
 
 +
# btrfs subvolume create ''/path/to/subvolume''
 +
 
 +
==== Listing subvolumes ====
 +
 
 +
To see a list of current subvolumes under {{ic|''path''}}:
 +
 
 +
# btrfs subvolume list -p ''path''
 +
 
 +
==== Deleting a subvolume ====
 +
 
 +
To delete a subvolume:
 +
 
 +
# btrfs subvolume delete ''/path/to/subvolume''
 +
 
 +
Attempting to remove the directory {{ic|''/path/to/subvolume''}} without using the above command will not delete the subvolume.
 +
 
 +
==== Mounting subvolumes ====
 +
 
 +
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]].
 +
 
 +
{{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|/}}.}}
 +
 
 +
{{Note|"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." [https://btrfs.wiki.kernel.org/index.php/Mount_options]  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.}}
 +
 
 +
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.
 +
 
 +
==== Changing the default sub-volume ====
 +
 
 +
The default sub-volume is mounted if no {{ic|1=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 subvolumes|listing]].
 +
 
 +
{{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].}}
 +
 
 +
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|<nowiki>subvol=/</nowiki>}} or {{ic|<nowiki>subvolid=5</nowiki>}} mount options [https://btrfs.wiki.kernel.org/index.php/SysadminGuide].
 +
 
 +
=== 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 [https://btrfs.wiki.kernel.org/index.php/Quota_support#Known_issues 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.
  
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:
+
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.
  
#  mkfs.btrfs -L ''mylabel'' -l 16k /dev/''partition''
+
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
  
Multiple devices can be entered to create a RAID. Supported RAID levels include RAID 0, RAID 1 and RAID 10. By default the metadata is mirrored and data is striped.
+
# btrfs qgroup show -reF <path>
  
# mkfs.btrfs [''options''] /dev/''part1'' /dev/''part2''
+
=== Commit Interval ===
  
=== Convert from Ext3/4 ===
+
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.
  
Boot from an install CD, then convert by doing:
+
LABEL=arch64 / btrfs defaults,noatime,compress=lzo,commit=120 0 0
  
# btrfs-convert /dev/''partition''
+
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}}.
  
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 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 your fstab accordingly if you use UUIDs.  {{ic|chroot}} into the system and rebuild the GRUB menu list (see [[Install from Existing Linux]] and [[GRUB]] articles).
+
=== SSD TRIM ===
 +
A Btrfs filesystem is able to free unused blocks from an SSD drive supporting the TRIM command.
  
To complete, delete the saved image, delete the sub-volume that image is on, then balance the drive to reclaim the space.
+
More information about enabling and using TRIM can be found in [[Solid State Drives#TRIM]].
  
# rm /ext2_saved/*
+
== Usage ==
# btrfs subvolume delete /ext2_saved
 
  
 
=== Displaying used/free space ===
 
=== Displaying used/free space ===
  
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 df, and then using btrfs fi df:
+
General linux userspace tools such as {{ic|/usr/bin/df}} will inaccurately report free space on a Btrfs partition. 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}}:
  
{{hc|# df -h /|
+
{{hc|$ df -h /|
 
Filesystem      Size  Used Avail Use% Mounted on
 
Filesystem      Size  Used Avail Use% Mounted on
 
/dev/sda3      119G  3.0G  116G  3% /
 
/dev/sda3      119G  3.0G  116G  3% /
 
}}
 
}}
  
{{hc|# btrfs fi df /|2=
+
{{hc|$ btrfs filesystem df /|2=
 
Data: total=3.01GB, used=2.73GB
 
Data: total=3.01GB, used=2.73GB
 
System: total=4.00MB, used=16.00KB
 
System: total=4.00MB, used=16.00KB
Line 68: Line 213:
 
}}
 
}}
  
Notice that {{ic|df -h}} reports 3.0GB used but {{ic|btrfs fi 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}}.
+
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}}.
 +
 
 +
{{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.}}
 +
 
 +
Another useful command to show a less verbose readout of used space is {{ic|btrfs filesystem show}}:
 +
 
 +
# btrfs filesystem show /dev/sda3
 +
 
 +
A more verbose command combining the information of {{ic|df}} and {{ic|show}} which directly links the free and used space is {{ic|btrfs filesystem usage}}. It is supposed to replace the {{ic|btrfs filesystem df}} command in the long run:
 +
 
 +
# btrfs filesystem usage /
 +
 
 +
{{Note|1=The {{ic|btrfs filesystem usage}} command does not currently work correctly with {{ic|RAID5/RAID6}} RAID levels.}}
  
Another useful command to show a less verbose readout of used space is {{ic|btrfs fi show}}:
+
=== Defragmentation ===
  
{{hc|# btrfs fi show /dev/sda3|
+
Btrfs supports online defragmentation through a [https://btrfs.wiki.kernel.org/index.php/Mount_options mount option]. To manually defragment your root, use:
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
+
# btrfs filesystem defragment -r /
}}
+
 
 +
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.
 +
 
 +
Defragmenting a file which has a COW copy (either a snapshot copy or one made with 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.
 +
 
 +
=== RAID ===
 +
 
 +
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.
 +
 
 +
{{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.}}
 +
 
 +
==== Scrub ====
 +
 
 +
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."
 +
 
 +
{{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.}}
 +
 
 +
===== Start manually =====
 +
 
 +
To start a (background) scrub on the filesystem which contains {{ic|/}}:
 +
 
 +
# btrfs scrub start /
 +
 
 +
To check the status of a running scrub:
 +
 
 +
# btrfs scrub status /
 +
 
 +
===== Start with a service or timer =====
 +
 
 +
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 the ''systemd-escape'' tool to escape a given string, see {{ic|systemd-escape(1)}} for examples.
 +
 
 +
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]].
 +
 
 +
==== 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." [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].
 +
 
 +
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].
 +
 
 +
# 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 [https://btrfs.wiki.kernel.org/index.php/SysadminGuide#Snapshots Btrfs Wiki SysadminGuide#Snapshots] for details.
 +
 
 +
To create a snapshot:
 +
 
 +
# btrfs subvolume snapshot ''source'' [''dest''/]''name''
 +
 
 +
To create a readonly snapshot add the {{ic|-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 {{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:
 +
 
 +
  # 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 {{ic|/backup}} above).
 +
 
 +
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:
 +
 
 +
  # btrfs send -p /root_backup /root_backup_new | btrfs receive /backup
 +
 
 +
Now a new subvolume named {{ic|root_backup_new}} will be present in {{ic|/backup}}.
  
== Limitations ==
+
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.
  
A few limitations should be known before trying.
+
=== Deduplication ===
  
=== Encryption ===
+
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.
  
Btrfs has no built-in encryption support (this may come in future), but you can encrypt the partition before running {{ic|mkfs.btrfs}}. See [[Dm-crypt with LUKS]].  
+
For an overview of available tools to deduplicate your Btrfs partition have a look at the [https://btrfs.wiki.kernel.org/index.php/Deduplication#Batch upstream Wiki entry].
  
If you've already created a Btrfs file system, you can also use something like [[EncFS]] or [[TrueCrypt]], though perhaps without some of Btrfs' features.
+
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].
  
=== Swap file ===
+
== Known issues ==
  
Btrfs does not support swap files. This is due to swap files requiring a function that Btrfs doesn't have for possibility of corruptions.<sup>[https://btrfs.wiki.kernel.org/index.php/FAQ#Does_btrfs_support_swap_files.3F link]</sup> A swap file can be mounted on a loop device with poorer performance but will not be able to hibernate. A ''systemd'' service file is available {{AUR|systemd-loop-swapfile}}.
+
A few limitations should be known before trying.
  
=== GRUB2 and core.img ===
+
=== Encryption ===
  
[[GRUB]] can boot Btrfs partitions however the module is larger than e.g. ext4 and the {{ic|core.img}} file made by ''grub-install'' may not fit between the MBR and the first partition. This can be solved by using GPT or by putting an extra 1 or 2 MB of free space before the first partition.
+
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]].  
  
If you get 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 your system should boot without an error.
+
Existing Btrfs file systems can use something like [[EncFS]] or [[TrueCrypt]], though perhaps without some of Btrfs' features.
  
== Features ==
+
=== Swap file ===
  
Various features are available and can be adjusted.
+
Btrfs does not yet support [[Swap#Swap_file|swap files]]. This is due to swap files requiring a function that Btrfs intentionally does not have for possibility of file system corruption [https://btrfs.wiki.kernel.org/index.php/FAQ#Does_btrfs_support_swap_files.3F]. Patches for swapfile support are already available [https://lkml.org/lkml/2014/12/9/718] and may be included in an upcoming kernel release. As an alternative 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}} to automate this.
  
=== Copy-On-Write (CoW) ===
+
=== TLP ===
  
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. As of Btrfs v0.20-rc1-253-g7854c8b users cannot tweak this without recompiling a [http://www.mail-archive.com/linux-btrfs@vger.kernel.org/msg26090.html patched version of fs/btrfs/disk-io.c]. On 01-Aug-2013, David Sterba submitted [http://www.mail-archive.com/linux-btrfs@vger.kernel.org/msg26116.html this patch] to make this a formal mount time tuneable.
+
Using TLP requires special precautions in order to avoid filesystem corruption. Refer to the [[TLP#Btrfs|according TLP section]] for more information.
  
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}}.
+
== Tips and tricks ==
  
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.
+
=== Partitionless Btrfs disk ===
You 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.
 
  
To disable CoW for single files/directories do:
+
{{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.}}
  
# chattr +C ''/dir/file''
+
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 [[#Creating a new file system|create a Btrfs filesystem]] on an existing [[partition]] that was created using another method. There are some limitations to partitionless single disk setups:
  
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.
+
* Cannot use different [[file systems]] for different [[fstab|mount points]].
 +
* 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]]. This also limits the use of hibernation/resume, which needs a swap area to store the hibernation image.
 +
* Cannot use [[UEFI]] to boot.
  
Likewise, to save space by forcing CoW when copying files use:
+
To overwrite the existing partition table with Btrfs, run the following command:
  
  # cp --reflink source dest
+
  # mkfs.btrfs /dev/sd''X''
  
As 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}}.
+
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.
  
=== Multi-device filesystem and RAID feature ===
+
Install the [[boot loader]] like you would for a data storage device with a [[Master Boot Record]]. See [[Syslinux#Manual install]] or  [[GRUB#Install to partition or partitionless disk]].
  
==== Multi-device filesystem ====
+
=== Ext3/4 to Btrfs conversion ===
  
When creating a ''btrfs'' filesystem, you can pass as many partitions or disk devices as you want to ''mkfs.btrfs''. The filesystem will be created across these devices. You can '''"'''pool'''"''' this way, multiple partitions or devices to get a big ''btrfs'' filesystem.
+
{{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.}}
  
You can also add or remove device from an existing btrfs filesystem (caution is mandatory).
+
Boot from an install CD, then convert by doing:
  
A multi-device ''btrfs'' filesystem (also called a btrfs volume) is not recognized until
+
  # btrfs-convert /dev/''partition''
  # btrfs device scan
 
has been run. This is the purpose of the ''btrfs'' mkinitcpio hook.
 
  
==== RAID features ====
+
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}}.
  
When creating multi-device filesystem, you can also specify to use RAID0, RAID1 or RAID10 across the devices you have added to 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.
+
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.
  
btrfs works in block-pairs for raid0, raid1, and raid10. This means:
+
# btrfs subvolume delete /ext2_saved
  
raid0 - block-pair stripped across 2 devices
+
Finally [[#Balance|balance]] the file system to reclaim the space.
  
raid1 - block-pair written to 2 devices
+
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.
  
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.
+
=== Checksum hardware acceleration ===
  
For example:
+
To verify if Btrfs checksum is hardware accelerated:
 +
{{hc|<nowiki>$ dmesg | grep crc32c</nowiki>|<nowiki>Btrfs loaded, crc32c=crc32c-intel</nowiki>}}
  
3 1TB disks in an md based raid1 yields a {{ic|/dev/md0}} with 1 TB free space and the ability to safely loose 2 disks without losing data.
+
If you see {{ic|<nowiki>crc32c=crc32c-generic</nowiki>}}, 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 {{ic|crc32c-intel}} into [[mkinitcpio.conf]] does ''not'' work.
3 1TB disks in a Btrfs volume with data=raid1 will allow the storage of approximately 1.5 TB 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.
+
=== Corruption recovery ===
  
=== Sub-volumes ===
+
''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:
  
One of the features of Btrfs is the use of sub-volumes. Sub-volumes are basically a named btree that holds files and directories. They have inodes inside the tree of tree roots and can have non-root owners and groups. Sub-volumes can optionally be given a quota of blocksAll of the blocks and file extents inside of sub-volumes are reference counted to allow snapshotting. This is similar to the dynamically expanding storage of a virtual machine that will only use as much space on a device as needed, eliminating several half-filled partitions.  One can also mount the sub-volumes with different mount options, giving more flexibility in security.
+
{{hc|/etc/mkinitcpio.conf|output=
 +
  BINARIES="/usr/bin/btrfs"
 +
}}
  
To create a sub-volume:
+
Regenerate the initial ramdisk using [[mkinitcpio]].
  
# btrfs subvolume create [''dest''/]
+
Then if there is a problem booting, the utility is available for repair.
  
For increased flexibility, install your system into a dedicated sub-volume, and, in the kernel boot parameters, use:
+
{{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.}}
  
rootflags=subvol=''whatever you called the subvol''
+
See the [https://btrfs.wiki.kernel.org/index.php/Btrfsck Btrfs Wiki page] for more information.
  
This makes system rollbacks possible.
+
=== Booting into snapshots ===
  
If using for the root partition, it is advisable to add '''crc32c''' (or '''crc32c-intel''' for Intel machines) to the modules array in {{ic|/etc/mkinitcpio.conf}}.
+
In order to boot into a snapshot you must specify the subvolume via a [[Kernel parameters#Configuration|kernel parameter]] using {{ic|1=rootflags=subvol=''/path/to/subvolume''}} and alter your {{ic|/etc/fstab}} to point to the same subvolume using {{ic|1=subvol=}}. Alternatively the subvolume can be specified with its id - retrievable with e.g. {{ic|btrfs subvolume list ''/root/path''}} - and {{ic|1=rootflags=subvolid=''objectid''}} as kernel parameter respectively {{ic|1=subvolid=''objectid''}} as mount option in {{ic|/etc/fstab}}.
  
=== Snapshots ===
+
If using GRUB you can automatically populate your boot menu with btrfs snapshots when regenerating the configuration file with the help of {{AUR|grub-btrfs}} or {{AUR|grub-btrfs-git}}.
  
To create a snapshot:
+
=== Use Btrfs subvolumes with systemd-nspawn ===
  
# btrfs subvolume snapshot ''source'' [''dest''/]''name''
+
See the [[Systemd-nspawn#Use Btrfs subvolume as container root]] and [[Systemd-nspawn#Use temporary Btrfs snapshot of container]] articles.
  
Snapshots are not recursive, this means that every subvolume inside subvolume will be an empty directory inside the snapshot.
+
== Troubleshooting ==
  
=== Defragmentation ===
+
See the [https://btrfs.wiki.kernel.org/index.php/Problem_FAQ Btrfs Problem FAQ] for general troubleshooting.
  
Btrfs supports online defragmentation. To defragment the metadata of the root folder do:
+
=== GRUB ===
  
# btrfs filesystem defragment /
+
==== Partition offset ====
  
This ''will not'' defragment the entire 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.
+
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.
  
To defragment the entire system verbosely do:
+
[[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.
  
# find / -xdev -type f -print -exec btrfs filesystem defrag '{}' \;
+
==== Missing root ====
  
=== Compression ===
+
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 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). Compression is enabled using the {{ic|1=compress=gzip}} or {{ic|1=compress=lzo}} mount options. Only files created or modified after the mount option is added will be compressed, so to fully benefit from compression it should be enabled during installation.
+
=== BTRFS: open_ctree failed ===
  
However, it can quite easily be applied to a subvolume using the defragment -czlib (or whichever algorithm you so choose) command (the same command above could be used, by adding the -czlib and such, to recursively apply). Also keep in mind that for future files to be compressed, a simple {{ic|chattr +c}} should be applied to some directories, so as to automatically compress new files as they come.
+
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}}:
  
After [[Beginners%27_Guide#Prepare_the_storage_drive|preparing the storage drive]], simply switch to another terminal ({{ic|Ctrl+Alt+number}}), and run the following command:
+
{{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
  
# mount -o remount,compress=lzo /dev/sd''XY'' /mnt/target
+
In some cases useful info is found in syslog - try dmesg|tail or so.
  
After the installation is finished, add {{ic|1=compress=lzo}} to the mount options of the root filesystem in {{ic|/etc/[[fstab]]}}.
+
You are now being dropped into an emergency shell.
 +
</nowiki>}}
  
=== Checkpoint Interval ===
+
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.
  
Starting with Linux 3.12, users are able to change the checkpoint interval from the default 30 s to any value by appending the '''commit''' mount flag in {{ic|/etc/fstab}} for the btrfs partition.
+
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]].
  
LABEL=arch64 / btrfs defaults,noatime,ssd,compress=lzo,commit=120 0 0
+
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].
  
=== Partitioning ===
+
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.
  
Btrfs can occupy an entire data storage device and replace the [[MBR]] or [[GPT]] partitioning schemes. You can use [[Btrfs#Sub-volumes|subvolumes]] to simulate partitions. There are some limitations to this approach if you use only a single disk:
+
See the [https://bbs.archlinux.org/viewtopic.php?id=189845 original forums thread] and {{Bug|42884}} for further information and discussion.
  
* You can not use different [[File_Systems|file systems]] for different [[fstab|mount points]].
+
=== btrfs check ===
* You can not 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]].
+
{{Warning|Since Btrfs is under heavy development, especially the {{ic|btrfs check}} command, it is highly recommended to create a '''backup''' and consult the following Btfrs documentation before executing {{ic|btrfs check}} with the {{ic|--repair}} switch.}}
* You can not use [[UEFI]] to boot.
 
  
To overwrite the existing partition table with Btrfs, run the following command:
+
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.
# mkfs.btrfs /dev/sd''X''
 
Do not specify {{ic|/dev/sda''X''}} or it will format an existing partition instead of replacing the entire partitioning scheme.
 
  
Install your [[Bootloaders|boot loader]] the same way you install it for a data storage device with a [[MBR|Master Boot Record]]. For example:
+
See [https://btrfs.wiki.kernel.org/index.php/Btrfsck Btrfsck] for more information.
# grub-install --recheck /dev/sd''X''
 
for [[Grub#Install_to_440-byte_MBR_boot_code_region|GRUB]].
 
  
 
== See also ==
 
== See also ==
  
* [https://btrfs.wiki.kernel.org/ Btrfs Wiki]
+
* '''Official site'''
* [https://btrfs.wiki.kernel.org/index.php/Problem_FAQ BTRFS Problem FAQ] - Official FAQ
+
** [https://btrfs.wiki.kernel.org/ Btrfs Wiki]
* [http://www.funtoo.org/wiki/BTRFS_Fun Funtoo Btrfs wiki entry] - Very well-written article
+
* '''Performance related'''
* [http://www.phoronix.com/scan.php?page=news_item&px=MTA0ODU Avi Miller presenting BTRFS] at SCALE 10x.  Jan/2012.
+
** [http://superuser.com/questions/432188/should-i-put-my-multi-device-btrfs-filesystem-on-disk-partitions-or-raw-devices Btrfs on raw disks?]
* [http://www.phoronix.com/scan.php?page=news_item&px=MTA4Mzc Summary of Chris Mason's talk from LFCS 2012]
+
** [http://comments.gmane.org/gmane.comp.file-systems.btrfs/19440 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)]
 +
** [https://btrfs.wiki.kernel.org/index.php/FAQ#Is_Btrfs_optimized_for_SSD.3F Is Btrfs optimized for SSDs?]
 +
** '''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_btrfs_311&num=1 Linux 3.11]
 +
*** [http://www.phoronix.com/scan.php?page=news_item&px=MTM0OTU Linux 3.9]
 +
*** [http://www.phoronix.com/scan.php?page=article&item=btrfs_linux37_mounts&num=1 Linux 3.7]
 +
*** [http://www.phoronix.com/scan.php?page=article&item=linux_btrfs_options&num=1 Linux 3.2]
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** [http://blog.erdemagaoglu.com/post/4605524309/lzo-vs-snappy-vs-lzf-vs-zlib-a-comparison-of Lzo vs. zLib]
 +
* '''Miscellaneous'''
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** [http://www.funtoo.org/wiki/BTRFS_Fun Funtoo Wiki Btrfs Fun]
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** [http://www.phoronix.com/scan.php?page=news_item&px=MTA0ODU Avi Miller presenting Btrfs] at SCALE 10x, January 2012.
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** [http://www.phoronix.com/scan.php?page=news_item&px=MTA4Mzc Summary of Chris Mason's talk] from LFCS 2012
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** [http://git.kernel.org/?p&#61;linux/kernel/git/torvalds/linux-2.6.git;a&#61;commit;h&#61;35054394c4b3cecd52577c2662c84da1f3e73525 Btrfs: stop providing a bmap operation to avoid swapfile corruptions] 2009-01-21
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** [http://marc.merlins.org/perso/btrfs/post_2014-03-22_Btrfs-Tips_-Doing-Fast-Incremental-Backups-With-Btrfs-Send-and-Receive.html Doing Fast Incremental Backups With Btrfs Send and Receive]

Latest revision as of 13:51, 24 March 2017

From Wikipedia:Btrfs:

Btrfs (B-tree file system, pronounced as "butter F S", "better F S", "b-tree F S", or simply by spelling it out) is a file system based on the copy-on-write (COW) principle, initially designed at Oracle Corporation for use in Linux. The development of Btrfs began in 2007, and by August 2014, the file system's on-disk format has been marked as stable.

From Btrfs Wiki:

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.
Warning: Btrfs has some features that are considered experimental. See the Btrfs Wiki's Status, Is Btrfs stable? and Getting started for more detailed information. See the #Known issues section.

Preparation

The official kernels linux and linux-lts include support for Btrfs. If you want to boot from a Btrfs file system, check if your boot loader supports Btrfs.

User space utilities are available by installing the btrfs-progs package.

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.

File system on a single device

To format a partition do:

# 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, RAID 6 mode of Btrfs is considered 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.
  • Some boot loaders such as Syslinux do not support multi-device file systems.

Multiple devices can be entered 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 as well as the manpage for mkfs.btrfs.

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

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: If the disks in your multi-disk array have different sizes, this may not use the full capacity of all drives. In order to utilize the full capacity of all disks, use -d single instead of -d raid0 -m raid1 (metadata mirrored, data not mirrored and not striped)
Note: Mounting such a filesystem may result in all but one of the according .device-jobs getting stuck and systemd never finishing startup due to a bug in handling this type of filesystem.

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 Wikipedia: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.

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.

Forcing CoW

To force copy-on-write when copying files use:

$ cp --reflink source dest 

This would only be required if CoW was disabled for the file to be copied (as implemented above). See the man page on cp for more details on the --reflink flag.

Compression

Btrfs supports transparent compression, meaning every file on the partition is automatically compressed. This not only reduces the size of files, but also improves performance, in particular if using the lzo algorithm, in some specific use cases (e.g. single thread with heavy file IO), while obviously harming performance on other cases (e.g. multithreaded and/or cpu intensive tasks with large file IO).

Compression is enabled using the compress=zlib or 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 btrfs filesystem defragment -calg command, where alg is either zlib or lzo. In order to re-compress the whole file system with lzo, run the following command:

# btrfs filesystem defragment -r -v -clzo /
Tip: Compression can also be enabled per-file without using the compress mount option; simply apply chattr +c to the file. When applied to directories, it will cause new files to be automatically compressed as they come.

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

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." [1]

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 under path:

# btrfs subvolume list -p path

Deleting a subvolume

To delete a subvolume:

# btrfs subvolume delete /path/to/subvolume

Attempting to remove the directory /path/to/subvolume without using the above command will not delete the subvolume.

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: "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." [2] 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.

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

Displaying used/free space

General linux userspace tools such as /usr/bin/df will inaccurately report free space on a Btrfs partition. It is recommended to use /usr/bin/btrfs to query a Btrfs partition. Below is an illustration of this effect, first querying using df -h, and then using btrfs filesystem df:

$ df -h /
Filesystem      Size  Used Avail Use% Mounted on
/dev/sda3       119G  3.0G  116G   3% /
$ btrfs filesystem df /
Data: total=3.01GB, used=2.73GB
System: total=4.00MB, used=16.00KB
Metadata: total=1.01GB, used=181.83MB

Notice that df -h reports 3.0GB used but 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 df -h.

Note: If you see an entry of type unknown in the output of btrfs filesystem df at kernel >= 3.15, this is a display bug. As of this patch, the entry means GlobalReserve, which is kind of a buffer for changes not yet flushed. This entry is displayed as unknown, single in RAID setups and is not possible to re-balance.

Another useful command to show a less verbose readout of used space is btrfs filesystem show:

# btrfs filesystem show /dev/sda3

A more verbose command combining the information of df and show which directly links the free and used space is btrfs filesystem usage. It is supposed to replace the btrfs filesystem df command in the long run:

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

Defragmentation

Btrfs supports online defragmentation through a mount option. 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 the systemd-escape tool to escape a given string, see systemd-escape(1) for examples.

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." [4] 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.

For an overview of available tools to deduplicate your Btrfs partition 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.

Swap file

Btrfs does not yet support swap files. This is due to swap files requiring a function that Btrfs intentionally does not have for possibility of file system corruption [5]. Patches for swapfile support are already available [6] and may be included in an upcoming kernel release. As an alternative a swap file can be mounted on a loop device with poorer performance but will not be able to hibernate. Install the package systemd-swap to automate this.

TLP

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

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.

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:

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.

Install the boot loader like you would for a data storage device with a Master Boot Record. See Syslinux#Manual install or GRUB#Install to partition or partitionless disk.

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

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 initial ramdisk using mkinitcpio.

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 you must specify the subvolume via a kernel parameter using rootflags=subvol=/path/to/subvolume and alter your /etc/fstab to point to the same subvolume using subvol=. Alternatively the subvolume can be specified with its id - retrievable with e.g. btrfs subvolume list /root/path - and rootflags=subvolid=objectid as kernel parameter respectively subvolid=objectid as mount option in /etc/fstab.

If using GRUB you can automatically populate your boot menu with btrfs snapshots when regenerating the configuration file with the help of grub-btrfsAUR 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.

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

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 following Btfrs 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 Btrfsck for more information.

See also