Btrfs/sv

Btrfs är filsystem för Linux som syftar till att införa avancerade funktioner jämsides feltolerans, självläkande egenskaper och enkel administration.]] Utvecklas kollektivt av Oracle, Red Hat, Fujitsu, Intel, SUSE, STRATO och många fler, btrfs är licensierad enligt GPL och är öppen för medverkan av vem som helst.

Funktioner
Ext4 är säkert och stabilt och kan hantera stora, omfattande filsystem, men varför byta? Medan det är sant att Btrfs fortfarande ses som experimentell och växer än i stabilitet, är dagen då Btrfs blir standardfilsystemet för Linux inom räckhåll. En del Linux-distributioner har redan börjat byta till det med deras nuvarande utgåvor. Btrfs har ett antal avancerade funktioner gemensamt med ZFS, vilket är det som gjorde ZFS-filsystemet så populärt inom BSD-distributioner och NAS-enheter.


 * Copy on Write (CoW) och snapshotting - gör inkrementella säkerhetskopior smärtfria, till och med för ett hett filsystem eller virtuella maskiner (VM).
 * Kontrollsummor på filnivå - metadata för vardera fil innehåller en kontrollsumma som används för att upptäcka och reparera fel.
 * Kompremering - filer kan aktivt komprimeras och dekomprimeras, vilket ökar läsprestandan.
 * Automatisk defragmentering - filsystemet rättas till av en bakgrundstråd medan de används.
 * Undervolymer - filsystemet kan dela ett enda pool-utrymme istället för att sättas in i egna partitioner.
 * RAID - Btrfs implementerar dess egna RAID, så att LVM eller inte krävs för att använda RAID. RAID 0, 1 och 10 stöds; RAID 5 och 6 anses vara ostabila.
 * Partitioner är frivilliga - Btrfs kan använda partitioner, men den har möjligheten att använda råa enheter direkt.
 * Datadeduplicering - där finns begränsat stöd för datadeduplicering, men deduplicering kommer så småningom att bli en standardfunktion i Btrfs. Detta gör det möjligt för Btrfs att spara på utrymmet genom att jämföra filer med hjälp av binära diff-filer.

Inom en snar framtid, kommer nya klusterfilsystem utnyttja Btrfs med dess copy-on-write och andra avancerade funktioner för dess object store. Ceph är ett lovande exempel på ett klusterfilsystem, och kan utnyttja Btrfs.

Linux-kärnan
Aktivera följande kärnalternativ för att aktivera stöd för Btrfs:

Emerge
-paketet omfattar de nödvändiga verktyg för att jobba med Btrfs-filsystemet.

Användning
Att skriva långa Btrfs-kommandon blir väldigt snabbt krångligt. Varje kommando (förutom inledande -kommandot) kan förkortas ned till en mycket kort uppsättning av instruktioner. Detta är användbart när man jobbar från kommandoraden och vill minska mängden skrivna tecken.

Exempelvis, för att defragmentera ett filsystem under, visar följande det långa kommandot:

Förkorta varje längre kommando efter -kommandot till dess kortaste, unika prefix. I det här sammanhanget, betyder unik att inget annat -kommando matchar kommandots förkortning. Den korta varianten av det ovan blir:

Inget annat -kommando börjar på ;   är den enda. Detsamma gäller för -underkommandot för  -kommandot.

Skapande
För att skapa ett Btrfs-filsystem på partitionen :

I exemplet ovan, ersätt  med det andra partitionsnumret och   med bokstaven från hårddisken som ska formateras. Till exempel, för att formatera den tredje partitionen av den första hårddisken, med Btrfs, kör:

Montera
Efter skapandet kan filsystem monteras på flera sätt:


 * - manuell montering.
 * - definera monteringspunkter i för automatisk montering på uppstart.
 * Removable media - monterar automatiskt på begäran (användbart för USB-enheter).
 * AutoFS - montera automatiskt när filsystemet är tillgängligt.

Konvertera ext*-filsystem
Det är fullt möjligt att konvertera ext2, ext3 och ext4-filsystem till Btrfs med hjälp av -verktyget.

The following instructions only support the conversion of filesystems that are unmounted. To convert the root partition, boot to a system rescue disk (SystemRescueCD works nicely) and run the conversion commands on the root partition.

First, be sure the mount point is unmounted:

Check the integrity of the filesystem using the appropriate tool. In the next example, the filesystem is ext4:

Use to convert the ext* formatted device into a Btrfs-formatted device:

Be sure to edit after the device has been formatted to change the filesystem column from ext4 to Btrfs:

Defragmentation
Another feature of Btrfs is online defragmentation. To defragment a root Btrfs filesystem, run:

The  mount option sets the default behavior to online defragmentation.

Compression
Btrfs supports transparent compression using the zlib, lzo, and zstd (v5.1.0) compression algorithms.

It is possible to compress specific files using the file attributes:

The  mount option sets the default behavior to compress all the newly created files. To re-compress the whole filesystem using lzo compression run the following command:

Depending on the CPU and disk performance, using lzo compression could improve the overall throughput.

As alternatives to lzo it is possible to use the zlib or zstd compression algorithms. Zlib is slower but has a higher compression ratio, whereas zstd has a good ratio between the two.

To force zlib compression across the whole filesystem:

Substitute zstd for zlib in the example above to activate zstd compression.

Compression level
Since kernel version 4.15.0, zlib compression can now be set by levels 1-9. Since kernel version 5.1.0 zstd can be set to levels 1-15. For example, to set zlib to maximum compression at mount time:

Or to set minimal compression:

Or adjust compression by remounting:

The compression level should be visible in, or by checking the most recent output using the following command:

Adjust fstab for compression
Once a drive has been remounted or adjusted to compress data, be sure to add the appropriate modifications to the file. In this example, zstd compression is set with a level of 9 at mount time:

Compression ratio and disk usage
The usual userspace tools for determining used and free space like and  may provide inaccurate results on a Btrfs partition due to inherent design differences in the way files are written compared to, for example, ext2/3/4.

It is therefore advised to use the du/df alternatives provided by the btrfs userspace tool. In addition, the tool found in the sys-fs/compsize package can be helpful in providing additional information regarding compression ratios and the disk usage of compressed files. The following are example uses of these tools for a btrfs partition mounted under.

Multiple devices (RAID)
Btrfs can be used with multiple block devices in order to create RAIDs. Using Btrfs to create filesystems that span multiple devices is much easier than creating using, since there is no initialization time needed for creation.

Btrfs handles data and metadata separately. This is important to keep in mind when using a multi-device filesystem. It is possible to use separate profiles for data and metadata block groups. For example, metadata could be configured across multiple devices in RAID1, while data could be configured to RAID5. This profile is possible when using three or more block devices, since RAID5 requires a minimum of 3 block devices.

This type of profile offers the benefit of redundancy for metadata on each device and striping for data across devices, which increases read speeds. The drawback of this profile is more space than necessary is used for metadata, and write speeds are reduced for data blocks, since RAID5 uses a parity bit.

Creation
The simplest method is to use the entirety of unpartitioned block devices to create a filesystem spanning multiple devices. For example, to create a filesystem in RAID1 mode across two devices:

Conversion
Converting between RAID profiles is possible with the sub-command. For example, say three block devices are presently configured for RAID1 and mounted at. It is possible to convert the data in this profile from RAID1 to RAID5 with the following command:

Conversion can be performed while the filesystem is online and in use. Possible RAID modes in btrfs include RAID0, RAID1, RAID5, RAID6, and RAID10. See the upstream Btrfs wiki for more information.

Addition
Additional devices can be added to existing multi device file systems. Follow the removal section below.

A riskier, but faster, alternative to safely removing a device is to shut the system down (or if the system supports hot swappable drives, at least unmount the filesystem), physically disconnect and remove the device that is to be replaced, substituted and connect the new device in it's place, power up the system (if necessary).

Note: Systems that will be power cycled will have the multi device filesystem fail to mount, since a device has been physically removed from the pool.

Once the system is booted, mount the multi-device filesystem with, then perform the following steps on adding a new device.

After the device has been re-added it is then necessary to re-balance the filesystem to be sure data is spanned across the newly added device:

By device path
Block devices (disks) can be removed from multi-device filesystems using the subcommand:

By device ID
Use the usage subcommand to determine the device IDs:

Next use the device ID to remove the device. In this case will be removed:

Resizing
btrfs partitions can be resized while online using the built-in resize subcommand. For example, to add 50 gigabytes of space to the rootfs:

The command can also fill all available space:

Subvolumes
As mentioned above in the features list, Btrfs can create subvolumes. Subvolumes can be used to better organize and manage data. They become especially powerful when combined with snapshots. Important distinctions must be made between Btrfs subvolumes and subvolumes created by Logical Volume Management. Btrfs subvolumes are not block level devices, they are POSIX file namespaces. They can be created at any location in the filesystem and will act like any other directory on the system with one difference: subvolumes can be mounted and unmounted. Subvolumes are nestable (subvolumes can be created inside other subvolumes), and easily created or removed.

Create
To create a subvolume, issue the following command inside a Btrfs filesystem's name space:

Replace  with the desired destination and subvolume name. For example, if a Btrfs filesystem exists at, a subvolume could be created inside it using the following command:

List
To see the subvolume(s) that have been created, use the  command followed by a Btrfs filesystem location. If the current directory is somewhere inside a Btrfs filesystem, the following command will display the subvolume(s) that exist on the filesystem:

If a Btrfs filesystem with subvolumes exists at the mount point created in the example command above, the output from the list command will look similar to the following:

Remove
All available subvolume paths in a Btrfs filesystem can be seen using the list command above.

Subvolumes can be properly removed by using the  command, followed by the path to the subvolume:

As above, replace  with the actual path to the subvolume to be removed. To delete the subvolume used in the examples above, the following command would be issued:

Snapshots
Snapshots are subvolumes that share data and metadata with other subvolumes. This is made possible by Btrfs' Copy on Write (CoW) ability. Snapshots can be used for several purposes, one of which is to create backups of file system structures at specific points in time.

If the root filesystem is Btrfs, it is possible to create a snapshot using the  commands:

The following small shell script can be added to a timed cron job to create a timestamped snapshot backup of a Btrfs formatted root filesystem. The timestamps can be adjusted to whatever is preferred by the user.

Mounting
A subvolume can be mounted in a location different from where it was created, or users can choose to not mount them at all. For example, a user could create a Btrfs filesystem in and create  and  subvolumes. The subvolumes could then be mounted at and, with the original top level subvolume left unmounted. This results in a configuration where the subvolumes' relative path from the top level subvolume is different from their actual path.

To mount a subvolume, perform the following command, where  is the relative path of the subvolume from the top level subvolume, obtainable through the   command:

Similarly, the filesystem tab can be updated to mount a Btrfs subvolume:

Filesystem check
With a failing disk or corrupted data, it may be necessary to run the a filesystem check. Typically filesystem check commands are handled through the prefix, but for btrfs filesystems, checks are handled via the  subcommand:

Multi device filesystem mount fails
After ungracefully removing one or more devices from a multi device filesystem, attempting to mount the filesystem will fail:

This type of mount failure could be caused by missing one or more devices from the multi device filesystem. Missing devices can be detected by using the subcommand. In the following example is one of the devices still connected to the multi device filesystem:

Missing device can be ungracefully dropped from the filesystem by using the following command:

Using with VM disk images
When using Btrfs with virtual machine disk images, it is best to disable copy-on-write on the disk images in order to speed up IO performance. This can only be performed on files that are newly created. It also possible to disable CoW on all files created within a certain directory. For example, using the command:

Clear the free space cache
It is possible to clear Btrfs' free space cache by mounting the filesystem with the  mount option. For example:

Btrfs hogging memory (disk cache)
When utilizing some of Btrfs' special abilities (like making many  copies or creating high amounts of snapshots), a lot of memory can be consumed and not freed fast enough by the kernel's inode cache. This issue can go undiscovered since memory dedicated to the disk cache might not be clearly visible in traditional system monitoring utilities. The utility (available as part of the  package) was specifically created to determine how much memory kernel objects are consuming:

If the inode cache is consuming too much memory, the kernel can be manually instructed to drop the cache by echoing an integer value to the file.

To be safe, and to help the kernel determine the maximum amount of freeable memory, be sure to run a before running the  commands below:

Most of the time Btrfs users will probably want to to reclaim just the slab objects (dentries and btrfs_inodes):

To clear the entire disk cache (slab objects and the page cache) use instead:

More information on kernel slabs can be found in this dedoimedo blog entry.

Mounting Btrfs fails, returning mount: unknown filesystem type 'btrfs'
The original solution by Tim on Stack Exchange inspired the following solution: build the kernel manually instead of using :

Btrfs root doesn't boot
Genkernel's initramfs as created with the command below doesn't load btrfs:

Compile support for btrfs in the kernel rather than as a module, or use to generate the initramfs.

External resources

 * https://wiki.debian.org/Btrfs - As described by the Debian wiki.
 * https://wiki.archlinux.org/index.php/Btrfs Btrfs article - As described by the Arch Linux wiki.
 * http://www.funtoo.org/BTRFS_Fun - BTRFS Fun on the Funtoo wiki.
 * http://marc.merlins.org/perso/btrfs/post_2014-05-04_Fixing-Btrfs-Filesystem-Full-Problems.html - Tips and tricks on fixing niche Btrfs filesystem problems in some situations.