Btrfs - btrfs

• Uses a subvolume per instance, image and snapshot, creating Btrfs snapshots when creating a new object.

• Btrfs can be used as a storage backend inside a container (nesting), so long as the parent container is itself on Btrfs. (But see notes about Btrfs quota via qgroups.)

• Btrfs supports storage quotas via qgroups. While Btrfs qgroups are hierarchical, 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. If adherence to strict quotas is a necessity users should be mindful of this and maybe consider using a zfs storage pool with refquotas.

• When using quotas it is critical to take into account that Btrfs extents are immutable so when blocks are written they end up in new extents and the old ones remain until all of its 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. This 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. Our recommendation is to not use VMs with Btrfs storage pools, but if you insist then please ensure that the instance root disk’s size.state property is set to 2x the size of the root disk’s size to allow all blocks in the disk image file to be rewritten without reaching the qgroup quota. You may also find that using the btrfs.mount_options=compress-force storage pool option avoids this scenario as 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 as this is a storage pool option it will affect all volumes on the pool.

Storage pool configuration

Key	Type	Default	Description
btrfs.mount_options	string	user_subvol_rm_allowed	Mount options for block devices
source	string	-	Path to block device or loop file or filesystem entry

Storage volume configuration

Key	Type	Condition	Default	Description
security.shifted	bool	custom volume	false	Enable id shifting overlay (allows attach by multiple isolated instances)
security.unmapped	bool	custom volume	false	Disable id mapping for the volume
size	string	appropriate driver	same as volume.size	Size of the storage volume
snapshots.expiry	string	custom volume	-	Controls when snapshots are to be deleted (expects expression like 1M 2H 3d 4w 5m 6y)
snapshots.pattern	string	custom volume	snap%d	Pongo2 template string which represents the snapshot name (used for scheduled snapshots and unnamed snapshots)
snapshots.schedule	string	custom volume	-	Cron expression (<minute> <hour> <dom> <month> <dow>), or a comma separated list of schedule aliases (@hourly, @daily, @midnight, @weekly, @monthly, @annually, @yearly)

Growing a loop backed Btrfs pool

LXD doesn’t let you directly grow a loop backed Btrfs pool, but you can do so with:

sudo truncate -s +5G /var/lib/lxd/disks/<POOL>.img
sudo losetup -c <LOOPDEV>
sudo btrfs filesystem resize max /var/lib/lxd/storage-pools/<POOL>/

(NOTE: For users of the snap, use /var/snap/lxd/common/mntns/var/snap/lxd/common/lxd/ instead of /var/lib/lxd/)

• LOOPDEV refers to the mounted loop device (e.g. /dev/loop8) associated with the storage pool image.

• The mounted loop devices can be found using the following command:

losetup -l