Btrfs - btrfs

Btrfs is a local file system based on the COW principle. COW means that data is stored to a different block after it has been modified instead of overwriting the existing data, reducing the risk of data corruption. Unlike other file systems, Btrfs is extent-based, which means that it stores data in contiguous areas of memory.

In addition to basic file system features, Btrfs offers RAID and volume management, pooling, snapshots, checksums, compression and other features.

To use Btrfs, make sure you have btrfs-progs installed on your machine.

Terminology

A Btrfs file system can have subvolumes, which are named binary subtrees of the main tree of the file system with their own independent file and directory hierarchy. A Btrfs snapshot is a special type of subvolume that captures a specific state of another subvolume. Snapshots can be read-write or read-only.

btrfs driver in Incus

The btrfs driver in Incus uses a subvolume per instance, image and snapshot. When creating a new entity (for example, launching a new instance), it creates a Btrfs snapshot.

Btrfs doesn’t natively support storing block devices. Therefore, when using Btrfs for VMs, Incus creates a big file on disk to store the VM. This approach is not very efficient and might cause issues when creating snapshots.

Btrfs can be used as a storage backend inside a container in a nested Incus environment. In this case, the parent container itself must use Btrfs. Note, however, that the nested Incus setup does not inherit the Btrfs quotas from the parent (see Quotas below).

Quotas

Btrfs supports storage quotas via qgroups. Btrfs qgroups are hierarchical, but new subvolumes will not automatically be added to the qgroups of their parent subvolumes. This means that users can trivially escape any quotas that are set. Therefore, if strict quotas are needed, you should consider using a different storage driver (for example, ZFS with refquota or LVM with Btrfs on top).

When using quotas, you must take into account that Btrfs extents are immutable. When blocks are written, they end up in new extents. The old extents remain until all their data is dereferenced or rewritten. This means that a quota can be reached even if the total amount of space used by the current files in the subvolume is smaller than the quota.

Note

This issue is seen most often when using VMs on Btrfs, due to the random I/O nature of using raw disk image files on top of a Btrfs subvolume.

Therefore, you should never use VMs with Btrfs storage pools.

If you really need to use VMs with Btrfs storage pools, set the instance root disk’s size.state property to twice the size of the root disk’s size. This configuration allows all blocks in the disk image file to be rewritten without reaching the qgroup quota. The btrfs.mount_options=compress-force storage pool option can also avoid this scenario, because a side effect of enabling compression is to reduce the maximum extent size such that block rewrites don’t cause as much storage to be double-tracked. However, this is a storage pool option, and it therefore affects all volumes on the pool.

Configuration options

The following configuration options are available for storage pools that use the btrfs driver and for storage volumes in these pools.

Storage pool configuration

btrfs.mount_options

Mount options for block devices

Key:	btrfs.mount_options
Type:	string
Default:	user_subvol_rm_allowed
Scope:	global

size

Size of the storage pool when creating loop-based pools (in bytes, suffixes supported, can be increased to grow storage pool)

Key:	size
Type:	string
Default:	auto (20% of free disk space, >= 5 GiB and <= 30 GiB)
Scope:	local

source

Path to an existing block device, loop file or Btrfs subvolume

Key:	source
Type:	string
Default:
Scope:	local

source.wipe

Wipe the block device specified in source prior to creating the storage pool

Key:	source.wipe
Type:	bool
Default:	false
Scope:	local

Tip

In addition to these configurations, you can also set default values for the storage volume configurations. See Configure default values for storage volumes.

Storage volume configuration

initial.gid

GID of the volume owner in the instance

Key:	initial.gid
Type:	int
Default:	same as volume.initial.gid or 0
Condition:	custom volume with content type filesystem

initial.mode

Mode of the volume in the instance

Key:	initial.mode
Type:	int
Default:	same as volume.initial.mode or 711
Condition:	custom volume with content type filesystem

initial.uid

UID of the volume owner in the instance

Key:	initial.uid
Type:	int
Default:	same as volume.initial.uid or 0
Condition:	custom volume with content type filesystem

security.shared

Enable sharing the volume across multiple instances

Key:	security.shared
Type:	bool
Default:	same as volume.security.shared or false
Condition:	custom block volume

security.shifted

Enable ID shifting overlay (allows attach by multiple isolated instances)

Key:	security.shifted
Type:	bool
Default:	same as volume.security.shifted or false
Condition:	custom volume

security.unmapped

Disable ID mapping for the volume

Key:	security.unmapped
Type:	bool
Default:	same as volume.security.unmapped or false
Condition:	custom volume

size

Size/quota of the storage volume

Key:	size
Type:	string
Default:	same as volume.size
Condition:	appropriate driver

snapshots.expiry

Controls when snapshots are to be deleted (expects an expression like 1M 2H 3d 4w 5m 6y)

Key:	snapshots.expiry
Type:	string
Default:	same as volume.snapshot.expiry
Condition:	custom volume

snapshots.expiry.manual

Controls when snapshots are to be deleted (expects an expression like 1M 2H 3d 4w 5m 6y)

Key:	snapshots.expiry.manual
Type:	string
Default:	same as volume.snapshot.expiry.manual
Condition:	custom volume

snapshots.pattern

Pongo2 template string that represents the snapshot name (used for scheduled snapshots and unnamed snapshots) [1]

Key:	snapshots.pattern
Type:	string
Default:	same as volume.snapshot.pattern or snap%d
Condition:	custom volume

snapshots.schedule

Cron expression (<minute> <hour> <dom> <month> <dow>), a comma-separated list of schedule aliases (@hourly, @daily, @midnight, @weekly, @monthly, @annually, @yearly), or empty to disable automatic snapshots (the default)

Key:	snapshots.schedule
Type:	string
Default:	same as volume.snapshot.schedule
Condition:	custom volume

Storage bucket configuration

To enable storage buckets for local storage pool drivers and allow applications to access the buckets via the S3 protocol, you must configure the core.storage_buckets_address server setting.

size

Size/quota of the storage bucket

Key:	size
Type:	string
Default:	same as volume.size
Condition:	appropriate driver

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[1]

The snapshots.pattern option takes a Pongo2 template string to format the snapshot name.

To add a time stamp to the snapshot name, use the Pongo2 context variable creation_date. Make sure to format the date in your template string to avoid forbidden characters in the snapshot name. For example, set snapshots.pattern to {{ creation_date|date:'2006-01-02_15-04-05' }} to name the snapshots after their time of creation, down to the precision of a second.

Another way to avoid name collisions is to use the placeholder %d in the pattern. For the first snapshot, the placeholder is replaced with 0. For subsequent snapshots, the existing snapshot names are taken into account to find the highest number at the placeholder’s position. This number is then incremented by one for the new name.