# Btrfs

Related articles

- File systems
- Mkinitcpio-btrfs
- Snapper
- dm-crypt/Encrypting an entire system#Btrfs subvolumes with swap

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.

## Partitionless Btrfs 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:

- 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. This also limits the use of hibernation/resume, which needs a swap area to store the hibernation image.
- Cannot use UEFI to boot.

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

    # mkfs.btrfs /dev/sdX

For example, use `/dev/sda` rather than `/dev/sda1`. The latter would format an existing partition instead of replacing the entire partitioning scheme.

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.

Warning: GRUB strongly discourages installation to a partitionless disk.

## File system creation

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

### Creating a new file system

#### 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:

- As of August 2016, the RAID 5, RAID 6 mode of Btrfs is considered fatally flawed, and shouldn't be used for "anything but testing with throw-away data." [1]
- 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.

### Ext3/4 to Btrfs conversion

Warning: As of mid-to-late 2015, there are many reports on the btrfs mailing list about incomplete/corrupt/broken conversions. The situation is improving as patches are being submitted, but proceed very carefully. Make sure you have working backups of any data you cannot afford to lose. See Conversion from Ext3 on the btrfs wiki.

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.

## 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." [2]

Each Btrfs file system has a top-level subvolume with ID 5. It can be mounted as `/` (by default), or another subvolume can be mounted instead.

See the following links for more details:

- Btrfs Wiki SysadminGuide#Subvolumes
- Btrfs Wiki Getting started#Basic Filesystem Commands
- 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 `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 easily 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 `/`.

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

##### Mount options

When mounting subvolumes with `subvol=` several mount options are available. For example, mount options that affect #Compression or #Copy-On-Write (CoW) can be used.

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.

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

#### 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.
Warning: Changing the default subvolume with `btrfs subvolume set-default` will make the top level of the filesystem inaccessible when the default subvolume is mounted . Reference: Btrfs Wiki Sysadmin Guide.

### 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,ssd,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 will automatically 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 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

The newest command to get information on free/used space of a is `btrfs filesystem usage`:

    # btrfs filesystem usage

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

### Defragmentation

This article or section needs expansion.

Reason: Caveat from [3] plus the solution. (Discuss in Talk:Btrfs#)

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 /

### 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 scrub for the subvolume mounted at root do:

    # btrfs scrub start /

To check the status of the scrub do:

    # 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.

    # 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 sub-volume inside sub-volume 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 an incremental backup and for tools that automate the process.

## Known issues

A few limitations should be known before trying.

### Encryption

Btrfs has no built-in encryption support, but this may come in 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 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.

### Linux-rt kernel

This article or section is out of date.

Reason: We're on 4.6.4. Is this still an issue? (Discuss in Talk:Btrfs#)

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.

## Tips and tricks

### 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 with GRUB

You can manually create a GRUB#GNU/Linux menu entry with the `rootflags=subvol=` argument. The `subvol=` mount options in `/etc/fstab` of the snapshot to boot into also have to be specified correctly.

Alternatively, you can automatically populate your GRUB menu with btrfs snapshots when regenerating the GRUB configuration file by using 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

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 `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.

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

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.

Note: 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.

### 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

- Official site
  - Btrfs Wiki
- 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

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