Difference between revisions of "Btrfs"

From ArchWiki
Jump to navigation Jump to search
Line 59: Line 59:
  # btrfs-convert /dev/''partition''
  # btrfs-convert /dev/''partition''
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.
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}}.
To complete, delete the sub-volume saved image is on, and finally [[#Balance|balance]] the file system to reclaim the space.
To complete, delete the sub-volume saved image is on, and finally [[#Balance|balance]] the file system to reclaim the space.

Revision as of 23:38, 27 January 2015


From Wikipedia:Btrfs

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.

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.


Btrfs is included in the default kernel and its tools (btrfs-progs) are available in the official repositories. GRUB, mkinitcpio, and Syslinux have support for Btrfs and require no additional configuration.

Additional packages

Tip: See Btrfs Wiki Getting Started for suggestions regarding running Btrfs effectively.

General administration of Btrfs

Creating a new file system

A Btrfs file system can either be newly created or have one converted.

To format a partition do:

# mkfs.btrfs -L mylabel /dev/partition
Note: As of this commit (November 2013), Btrfs default blocksize is 16KB.

To use a larger blocksize for data/meta data, specify a value for the leafsize via the -l switch as shown in this example using 16KB blocks:

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

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 Using Btrfs with Multiple Devices for more information about how to create a Btrfs RAID volume.

# mkfs.btrfs [options] /dev/part1 /dev/part2

Convert from Ext3/4

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.

To complete, delete the sub-volume saved image is on, and finally balance the file system to reclaim the space.

# btrfs subvolume delete /ext2_saved

Mount options

Warning: Specific mount options can disable safety features and increase the risk of complete file system corruption.

See Btrfs Wiki Mount options and Btrfs Wiki Gotchas for more information.

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.

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.


  • Linux 3.15
    • Btrfs on a SSD for system installation and an emphasis on maximizing performance.
    • Btrfs on a HDD for archival purposes with an emphasis on maximizing space.

Displaying used/free space

General linux userspace tools such as /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 /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
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


A few limitations should be known before trying.


Btrfs has no built-in encryption support (this may come in future); users can encrypt the partition before running mkfs.btrfs. See dm-crypt.

Existing Btrfs file system, 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 doesn't have for possibility of file system corruption [1]. Patches for swapfile support are already available [2] 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 from the official repositories to automate this.

Linux-rt kernel

As of version 3.14.12_rt9, the linux-rt kernel does not boot with the Btrfs file system. This is due to the slow development of the rt patchset.


Various features are available and can be adjusted.

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 is tuneable using mount options (see below)

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.

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.

One can disable CoW for the entire block device by mounting it with nodatacow option. However, this will disable CoW for the entire file system.

Note: nodatacow will only affect newly created file. CoW may still happen for existing file.

To disable CoW for single files/directories do:

$ chattr +C /dir/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.
Tip: In accordance with the note above, you can use the following trick to disable CoW 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.

Likewise, to save space by forcing CoW when copying files use:

$ cp --reflink source dest 

As dest file is changed, only those blocks that are changed from source will be written to the disk. One might consider aliasing cp to cp --reflink=auto.

Multi-device filesystem and RAID feature

See Using Btrfs with Multiple Devices for suggestions.

Multi-device filesystem

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.

One can also add or remove device from an existing btrfs filesystem (caution is mandatory).

RAID features

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.

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 works in block-pairs for raid0, raid1, and raid10. This means:

raid0 - block-pair striped across 2 devices

raid1 - block-pair written to 2 devices

The raid level can be changed while the disks are online using the btrfs balance command:

# btrfs balance -m convert=RAIDLEVEL -d convert=RAIDLEVEL /path/to/mount

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.

For example:

  • Three 1TB disks in an md based raid1 yields a /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 the BTRFS FAQ for more info.


See the following links for more details:

Creating sub-volumes

To create a sub-volume:

# btrfs subvolume create subvolume-name

Listing sub-volumes

To see a list of current sub-volumes:

# btrfs subvolume list -p .

Setting a default sub-volume

Warning: Changing the default subvolume with btrfs subvolume default will make the top level of the filesystem inaccessible, except by use of the subvolid=0 mount option. Reference: Btrfs Wiki Sysadmin Guide.

The default sub-volume is mounted if no subvol= mount option is provided.

# btrfs subvolume set-default subvolume-id /.


# btrfs subvolume list .
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 .


# btrfs subvolume set-default 0 .


See Btrfs Wiki SysadminGuide#Snapshots for details.

To create a snapshot:

# btrfs subvolume snapshot source [dest/]name

Snapshots are not recursive. Every sub-volume inside sub-volume will be an empty directory inside the snapshot.

Installing with root on btrfs subvolume


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

Reason: Setting rootvol as the default subvolume (see #Setting a default sub-volume) would be much simpler, for both installation and booting. (Discuss in Talk:Btrfs#)
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.)

For increased flexibility, install the system into a dedicated sub-volume. 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 
# 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.

# cd /mnt
# btrfs subvolume create rootvol
# cd /
# umount /mnt

Mount the subvolume.

# mount -o subvol=rootvol /dev/sda1 /mnt

From here continue the installation process from the pacstrap step, as per the Installation guide or Beginners' guide.

Additional considerations:

  • 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: rootflags=subvol=subvolume-name. It is still necessary to add the standard root parameter with root=/dev/sda1.
  • It is advisable to add crc32c (or crc32c-intel for Intel machines) to the modules array in /etc/mkinitcpio.conf
Note: Child sub-volumes are automatically mounted by Btrfs when the parent sub-volume is mounted.


Btrfs supports online defragmentation. To defragment the metadata of the root folder do:

# btrfs filesystem defragment /

This will not defragment the entire file system. For more information read this page on the btrfs wiki.

To defragment the entire file system verbosely:

# btrfs filesystem defragment -r -v /
Note: The command above will defragment only file data. To defragment directory metadata for every directory in the file system, run this command:
# find / -xdev -type d -print -exec btrfs filesystem defragment '{}' \;


Btrfs supports transparent compression, which means every file on the partition is automatically compressed. This not only reduces the size of those 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, set the compress option after preparing the storage drive. Simply switch to another terminal (Ctrl+Alt+number), and run the following command:

# mount -o remount,compress=lzo /mnt/target

After the installation is finished, add compress=lzo to the mount options of the root file system in fstab.

Checkpoint interval

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 option in /etc/fstab for the btrfs partition.

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


Btrfs can occupy an entire data storage device and replace the MBR or GPT partitioning schemes. One can use subvolumes to simulate partitions. There are some limitations to this approach in single disk setups:

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

# mkfs.btrfs /dev/sdX

Do not specify /dev/sdaX or it will format an existing partition instead of replacing the entire partitioning scheme.

Install the boot loader in a like fashion to installing it for a data storage device with a Master Boot Record. For example:

# grub-install --recheck /dev/sdX

for GRUB.

Warning: Using the 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.


See Btrfs Wiki Glossary.

# btrfs scrub start /
# btrfs scrub status /
Warning: The running scrub process will prevent the system from suspending, see this thread for details.

If running the scrub as a systemd service, use 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.


See Upstream FAQ page.

Since btrfs-progs-3.12 balancing is a background process - see man 8 btrfs-balance for full description.

# btrfs balance start /
# btrfs balance status /



Partition offset

Note: 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 corg.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 adding btrfs to the MODULES array. Then regenerate the initramfs with mkinitcpio -p linux (adjust the preset if needed) and reboot.

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

See also