Export translations

Settings

Group

Language

Format

Export for off-line translation

Export in native format

<languages/>
Btrfs  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 since August 2014 the file system's on-disk format has been marked as stable.

The Funtoo Linux project recommends btrfs as a next-generation filesystem, particularly for use in production.

Btrfs is intended to address the lack of pooling, snapshots, checksums, and integral multi-device spanning in Linux file systems.

It is easy to set up and use btrfs. In this simple introduction, we're going to set up btrfs under Funtoo Linux using an existing {{c|debian-sources}} or {{c|debian-sources-lts}} kernel, like the one that comes pre-built for you with Funtoo Linux, and we will also be using our btrfs storage pool for storing data that isn't part of the Funtoo Linux installation itself. Funtoo Linux will boot from a non-btrfs filesystem, and as part of the initialization process will initialize our btrfs storage and mount it at the location of our choice.

== Installation ==

Enabling btrfs support is as simple as enabling the btrfs mix-in and running a world update:

{{console|body=
# ##i##epro mix-in +btrfs
# ##i##emerge -uDN @world
}}

Btrfs is now ready for use.

== Btrfs Concepts ==

Btrfs can be used to manage the physical disks that it uses, and physical disks are added to a Btrfs volume. Then, BTRFS can create subvolumes from the volume on which files can be stored.

Unlike traditional Linux filesystems, btrfs filesystems will allocate storage on-demand from the underlying volume.

In the btrfs world, the word volume corresponds to a storage pool ([[ZFS]]) or a volume group (LVM).

* ''devices'' - one or multiple underlying physical volumes.
* ''volume'' - one large storage pool comprised of all space of the devices and can support different redundancy levels
* ''subvolumes'' - these are what get mounted and you store files in.
* ''snapshots'' - a read-only copy of a subvolume at a given point in time and/or read-write copy of a ''subvolume'' in time (aka clone).

== Creating a Volume ==

To create a basic btrfs volume, you will need an extra empty disk. Perform the following steps:

{{console|body=
# ##i## mkfs.btrfs /dev/sdxy
btrfs-progs v4.17.1 
See http://btrfs.wiki.kernel.org for more information.

Detected a SSD, turning off metadata duplication.  Mkfs with -m dup if you want to force metadata duplication.
Performing full device TRIM /dev/sdj (223.57GiB) ...
Label:              (null)
UUID:               d6bcba6e-8fd5-41fc-9bb4-79628c5c928c
Node size:          16384
Sector size:        4096
Filesystem size:    223.57GiB
Block group profiles:
  Data:             single            8.00MiB
  Metadata:         single            8.00MiB
  System:           single            4.00MiB
SSD detected:       yes
Incompat features:  extref, skinny-metadata
Number of devices:  1
Devices:
   ID        SIZE  PATH
    1   223.57GiB  /dev/sdxy
}}

{{c|/dev/sdxy}} should be an unused disk. You may need to use the following command if this disk contains any pre-existing data on it:

Now you can mount the created volume as you would mount any other linux filesystem.

{{console|body=
# ##i## mkdir /mnt/btrfs-top-level
# ##i## mount /dev/sdxy /mnt/btrfs-top-level
# ##i## mount
...
/dev/sdxy on /mnt/btrfs-top-level type btrfs (rw,relatime,ssd,space_cache,subvolid=5,subvol=/)
}}

{{Important|It is recommended that nothing is stored directly on this top-level volume (ID 5) root directory.}}

== Creating Subvolumes ==

Btrfs has a concept of subvolumes. Subvolume is an independently mountable POSIX filetree (but not a block device). There are several basic schemas to layout subvolumes (including snapshots) as well as mixtures thereof.

Lets create children of the top level subvolume (ID 5). We will have:
* {{c|@data}} - it will serve as mountable {{c|/data}}
* {{c|.snapshots}} - here snapshots will be stored

{{console|body=
# ##i## cd /mnt/btrfs-top-level
# ##i## btrfs subvolume create @data
# ##i## btrfs subvolume create .snapshots
# ##i## btrfs subvolume list /mnt/btrfs-top-level
ID 256 gen 322338 top level 5 path @data
ID 257 gen 322275 top level 5 path .snapshots
}}

== The default Subvolume ==

{{Note|Changing the default subvolume with {{c|btrfs subvolume default}} will make the top level of the filesystem accessible only when {{c|subvol}} or {{c|subvolid}} mount options are specified}}

When btrfs block device is mounted without specifying a subvolume the default one is used. To check default subvolume run
{{console|body=
# ##i## btrfs subvolume get-default /mnt/btrfs-top-level
ID 5 (FS_TREE)
}}

For the convenience lets make {{c|@data}} subvolume as the default one. It's good to double check the subvolume ID first. Either {{c|btrfs subvolume list}} or {{c|btrfs subvolume show}} can be used for that

{{console|body=
# ##i## btrfs subvolume show /mnt/btrfs-top-level/@data
...
	Subvolume ID: 		256
}}

Now you can make this subvolume as a default one
{{console|body=
# ##i## btrfs subvolume set-default 256 /mnt/btrfs-top-level
# ##i## btrfs subvolume get-default /mnt/btrfs-top-level
ID 256 gen 322336 top level 5 path @data
}}

At this point you can stop working on the top level subvolume (ID 5) and instead mount directly {{c|@data}} subvolume.

{{console|body=
# ##i## cd /mnt
# ##i## umount /mnt/btrfs-top-level
# ##i## mkdir /data
# ##i## mount /dev/sdxy /data
}}

== Nested Subvolumes ==

{{Note|Nested subvolumes are not going to be a part of snapshots created from their parent subvolume. So one typical reason is to exclude certain parts of the filesystem from being snapshot.}}

Lets create a separate nested subvolume for {{c|/data/independent}}.

{{console|body=
# ##i## btrfs subvolume create /data/independent
# ##i## btrfs subvolume list /data
ID 258 gen 161 top level 256 path independent
}}

Usually you will want to "split" areas which are "complete" and/or "consistent" in themselves. Examples for this more-fine grained partitioning could be {{c|/var/log}}, {{c|/var/www}} or {{c|/var/lib/postgresql}}.

== /etc/fstab ==

To automatically mount the {{c|@data}} subvolume after reboot you need to modify {{c|/etc/fstab}}

{{file|name=/etc/fstab|desc=fstab for btrfs|body=
/dev/sdxy	/data	btrfs	subvolid=256,defaults	0 0
}}

{{Warning|According to [https://btrfs.readthedocs.io/en/latest/Administration.html#mount-options btrfs docs] most mount options apply to the whole filesystem and only options in the first mounted subvolume will take effect. This means that (for example) you can't set per-subvolume {{c|nodatacow}}, {{c|nodatasum}}, or {{c|compress}}.}}

Now lets verify if this changes were correct
{{console|body=
# ##i## cd /
# ##i## umount /data
# ##i## mount /data
# ##i## ls /data
independent
}}

Did you just notice that although we mounted our {{c|@data}} subvolume the nested subvolume {{c|@data/independent}} is also present?

== Snapshots ==

For the purpose of checking out this cool btrfs feature lets populate our filesystem with some example data first

{{console|body=
# ##i## echo 'btrfs' > /data/foo.txt
# ##i## echo 'fun' > /data/independent/bar.txt
}}

As you probably remember on the top level (next to {{c|@data}} subvolume) you've also created the {{c|.snapshots}} subvolume. You can mount it now to create some snapshots
{{console|body=
# ##i## mkdir /mnt/snapshots
# ##i## mount /dev/sdxy /mnt/snapshots -o subvolid=257
}}

A snapshot is a subvolume like any other, with given initial content. By default, snapshots are created read-write. File modifications in a snapshot do not affect the files in the original subvolume. Lets create a read-write snapshot for {{c|/data}} and read-only snapshot for {{c|/data/independent}}

{{console|body=
# ##i## btrfs subvolume snapshot /data /mnt/snapshots/data_$(date -u -Iseconds)
Create a snapshot of '/data' in '/mnt/snapshots/data_2022-08-30T22:04:57+00:00'
# ##i## btrfs subvolume snapshot -r /data/independent /mnt/snapshots/independent_$(date -u -Iseconds)
Create a readonly snapshot of '/data/independent' in '/mnt/snapshots/independent_2022-08-30T22:05:29+00:00'
}}

Once again, nested subvolumes are not going to be a part of snapshots created from their parent subvolume. So you shouldn't be surprised when you compare the contents of the {{c|/data}} vs the contents of the {{c|/mnt/snapshots}}

{{console|body=
# ##i## tree /data
/data
├── foo.txt
└── independent
    └── bar.txt
# ##i## tree /mnt/snapshots
/mnt/snapshots
├── data_2022-08-30T22:04:57+00:00
│   └── foo.txt
└── independent_2022-08-30T22:05:29+00:00
    └── bar.txt
}}

At this point you might be interested in [https://btrfs.readthedocs.io/en/latest/Send-receive.html send and receive btrfs features].

{{Note|According to [https://btrfs.readthedocs.io/en/latest/Subvolumes.html btrfs docs] a snapshot is not a backup: snapshots work by use of BTRFS copy-on-write behaviour. A snapshot and the original it was taken from initially share all of the same data blocks. If that data is damaged in some way (cosmic rays, bad disk sector, accident with dd to the disk), then the snapshot and the original will both be damaged.}}

== Wrap up ==

{{Important|It is recommended to run {{c|btrfs scrub}} once in a while. E.g. every month}}

Scrub is the online check and repair functionality that verifies the integrity of data and metadata, assuming the tree structure is fine. You can run it on a mounted file system; it runs as a background process during normal operation.

To start a (background) scrub on the filesystem which contains {{c|/data}} run

{{console|body=
# ##i## btrfs scrub start /data
scrub started on /data, fsid 40f8b94f-07ee-4f7e-beb1-8e686abc246d (pid=5525)
}}

To check the status of a running scrub

{{console|body=
# ##i## btrfs scrub status /data
UUID:             40f8b94f-07ee-4f7e-beb1-8e686abc246d
Scrub started:    Tue Aug 30 00:38:54 2022
Status:           running
Duration:         0:00:15
Time left:        0:00:34
ETA:              Tue Aug 30 00:39:44 2022
Total to scrub:   149.06GiB
Bytes scrubbed:   44.79GiB  (30.04%)
Rate:             2.99GiB/s
Error summary:    no errors found
}}

You should now be at the point where you can begin to use btrfs for a variety of tasks. While there is a lot more to btrfs than what is covered in this short introduction, you should now have a good understanding of the fundamental concepts on which btrfs is based.

[[Category:Btrfs]]
[[Category:Filesystems]]
[[Category:HOWTO]]
[[Category:Official Documentation]]

Export translations

Navigation menu

Search