Btrfs - ArchWiki

# Btrfs

Related articles

- File systems
- Btrfs - Tips and tricks
- Mkinitcpio-btrfs

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.

Warning:

- Btrfs has some features that are considered experimental. See the Btrfs Wiki's Stability status, Is Btrfs stable, and Getting started for more detailed information.
- Beware of the limitations.

## Installation

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

- btrfs-progs includes btrfsck, a tool that can fix errors on Btrfs filesystems.
- mkinitcpio-btrfs enables roll-back abilities.
- btrfs-progs-git for nightly

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/metadata, 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.

#### Examples

- Linux 3.15
  - Btrfs on a SSD for system installation and an emphasis on maximizing performance.

        noatime,discard,ssd,autodefrag,compress=lzo,space_cache

  - Btrfs on a HDD for archival purposes with an emphasis on maximizing space.

        noatime,autodefrag,compress-force=lzo,space_cache

### 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

## Limitations

A few limitations should be known before trying.

### Encryption

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.

## Features

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 single 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 a 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 metadata. By default, metadata 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 start -mconvert=RAIDLEVEL -dconvert=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.

### Sub-volumes

See the following links for more details:

- Btrfs Wiki SysadminGuide#Subvolumes
- Btrfs Wiki Getting started#Basic Filessystem Commands
- Btrfs Wiki Trees

#### 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 /.

Example:

    # 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 .

Reset:

    # btrfs subvolume set-default 0 .

#### Snapshots

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

This article or section is a candidate for moving to [[]].

Notes: Does not fit into #Features section, maybe we should start new one... (Discuss)

The factual accuracy of this article or section is disputed.

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

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.

### Defragmentation

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

    # 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 '{}' \;

### 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, 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

### Partitioning

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

- Cannot use different file systems for different mount points.
- Cannot use swap area as Btrfs does not support swap files and there is no place to create swap partition.
- Cannot use UEFI to boot.

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.

### Scrub

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.

### Balance

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 /

## Troubleshooting

### GRUB

#### 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

- Official site
  - Btrfs Wiki
  - Btrfs Wiki Glossary
- Official FAQs
  - Btrfs Wiki FAQ
  - Btrfs Wiki Problem FAQ
- Btrfs pull requests
  - 3.14
  - 3.13
  - 3.12
  - 3.11
- Performance related
  - Btrfs on raw disks?
  - Varying leafsize and nodesize in Btrfs
  - Btrfs support for efficient SSD operation (data blocks alignment)
  - Is Btrfs optimized for SSDs?
  - Phoronix mount option benchmarking
    - Linux 3.14
    - Linux 3.11
    - Linux 3.9
    - Linux 3.7
    - Linux 3.2
  - Lzo vs. zLib
- Miscellaneous
  - Funtoo Wiki Btrfs Fun
  - Avi Miller presenting Btrfs at SCALE 10x, January 2012.
  - Summary of Chris Mason's talk from LFCS 2012
  - Btrfs: stop providing a bmap operation to avoid swapfile corruptions 2009-01-21
  - Doing Fast Incremental Backups With Btrfs Send and Receive

Category:

- File systems