Btrfs - `btrfs`#

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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 LXD#

The btrfs driver in LXD uses a subvolume per instance, image and snapshot. When creating a new object (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, LXD 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 LXD environment. In this case, the parent container itself must use Btrfs. Note, however, that the nested LXD 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#

Key	Type	Default	Description
`btrfs.mount_options`	string	`user_subvol_rm_allowed`	Mount options for block devices
`size`	string	auto (20% of free disk space, >= 5 GiB and <= 30 GiB)	Size of the storage pool when creating loop-based pools (in bytes, suffixes supported)

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#

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

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 (see Server configuration).

Key	Type	Condition	Default	Description
`size`	string	appropriate driver	same as `volume.size`	Size/quota of the storage bucket