[[chapter_btrfs]]
BTRFS
-----
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BTRFS is a modern copy on write file system natively supported by the Linux
kernel, implementing features such as snapshots, built-in RAID and self healing
via checksums for data and metadata. Starting with {pve} 7.0, BTRFS is
introduced as optional selection for the root file system.

.General BTRFS advantages

* Main system setup almost identical to the traditional ext4 based setup

* Snapshots

* Data compression on file system level

* Copy-on-write clone

* RAID0, RAID1 and RAID10

* Protection against data corruption

* Self healing

* natively supported by the Linux kernel

* ...

.Caveats

* RAID levels 5/6 are experimental and dangerous

Installation as Root File System
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

When you install using the {pve} installer, you can choose BTRFS for the root
file system. You need to select the RAID type at installation time:

[horizontal]
RAID0:: Also called ``striping''. The capacity of such volume is the sum
of the capacities of all disks. But RAID0 does not add any redundancy,
so the failure of a single drive makes the volume unusable.

RAID1:: Also called ``mirroring''. Data is written identically to all
disks. This mode requires at least 2 disks with the same size. The
resulting capacity is that of a single disk.

RAID10:: A combination of RAID0 and RAID1. Requires at least 4 disks.

The installer automatically partitions the disks and creates an additional
subvolume at `/var/lib/pve/local-btrfs`.  In order to use that with the {pve}
tools, the installer creates the following configuration entry in
`/etc/pve/storage.cfg`:

----
dir: local
	path /var/lib/vz
	content iso,vztmpl,backup
	disable

btrfs: local-btrfs
	path /var/lib/pve/local-btrfs
	content iso,vztmpl,backup,images,rootdir
----

This explicitly disables the default `local` storage in favor of a btrfs
specific storage entry on the additional subvolume.

The `btrfs` command is used to configure and manage the btrfs file system,
After the installation, the following command lists all additional subvolumes:

----
# btrfs subvolume list /
ID 256 gen 6 top level 5 path var/lib/pve/local-btrfs
----

BTRFS Administration
~~~~~~~~~~~~~~~~~~~~

This section gives you some usage examples for common tasks.

Creating a BTRFS file system
^^^^^^^^^^^^^^^^^^^^^^^^^^^^

To create BTRFS file systems, `mkfs.btrfs` is used. The `-d` and `-m` parameters
are used to set the profile for metadata and data respectively. With the
optional `-L` parameter, a label can be set.

Generally, the following modes are supported: `single`, `raid0`, `raid1`,
`raid10`.

Create a BTRFS file system on `/dev/sdb1`

----
 # mkfs.btrfs -m single -d single -L My-Storage /dev/sdb1
----

Or create a RAID1 on `/dev/sdb1` and `/dev/sdc1`

----
 # mkfs.btrfs -m raid1 -d raid1 -L My-Storage /dev/sdb1 /dev/sdc1
----

This can then be mounted or used in `/etc/fstab` like any other mount point.

For example

----
 # mkdir /my-storage
 # mount /dev/sdb1 /my-storage
----

Creating a subvolume
^^^^^^^^^^^^^^^^^^^^

Creating a subvolume links it to a path in the btrfs file system, where it will
appear as a regular directory.

----
# btrfs subvolume create /some/path
----

Afterwards `/some/path` will act like a regular directory.

Deleting a subvolume
^^^^^^^^^^^^^^^^^^^^

Contrary to directories removed via `rmdir`, subvolumes do not need to be empty
in order to be deleted via the `btrfs` command.

----
# btrfs subvolume delete /some/path
----

Creating a snapshot of a subvolume
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

BTRFS does not actually distinguish between snapshots and normal subvolumes, so
taking a snapshot can also be seen as creating an arbitrary copy of a subvolume.
By convention, {pve} will use the read-only flag when creating snapshots of
guest disks or subvolumes, but this flag can also be changed later on.

----
# btrfs subvolume snapshot -r /some/path /a/new/path
----

This will create a read-only "clone" of the subvolume on `/some/path` at
`/a/new/path`. Any future modifications to `/some/path` cause the modified data
to be copied before modification.

If the read-only (`-r`) option is left out, both subvolumes will be writable.

Enabling compression
^^^^^^^^^^^^^^^^^^^^

By default, BTRFS does not compress data. To enable compression, the `compress`
mount option can be added. Note that data already written will not be compressed
after the fact.

By default, the rootfs will be listed in `/etc/fstab` as follows:

----
UUID=<uuid of your root file system> / btrfs defaults 0 1
----

You can simply append `compress=zstd`, `compress=lzo`, or `compress=zlib` to the
`defaults` above like so:

----
UUID=<uuid of your root file system> / btrfs defaults,compress=zstd 0 1
----

This change will take effect after rebooting.