Btrfs - btrfs
Description: 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...

Hide navigation sidebar
Hide table of contents sidebar
Skip to content
Back to top
View this page
Toggle table of contents sidebar

# Btrfs - `btrfs`

▶ Watch on YouTube

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 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, 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. Setting the `btrfs.mount_options` storage pool option to `compress-force` 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`

`size`

Size of the storage pool (for loop-based pools)

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

When creating loop-based pools, specify the size in bytes (suffixes are supported). You can increase the size to grow the storage pool.

The default (`auto`) creates a storage pool that uses 20% of the free disk space, with a minimum of 5 GiB and a maximum of 30 GiB.

`source`

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

Key: | `source`
Type: | string

`source.wipe`

Whether to wipe the block device before creating the pool

Key: | `source.wipe`
Type: | bool
Default: | `false`

Set this option to `true` to wipe the block device specified in `source` prior to creating the storage pool.

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

`security.shifted`

Enable ID shifting overlay

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

Enabling this option allows attaching the volume to multiple isolated instances.

`security.unmapped`

Disable ID mapping for the volume

Key: | `security.unmapped`
Type: | bool
Default: | same as `volume.security.unmappped` 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`

When snapshots are to be deleted

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

Specify an expression like `1M 2H 3d 4w 5m 6y`.

`snapshots.pattern`

Template for the snapshot name

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

You can specify a naming template that is used for scheduled snapshots and unnamed snapshots.

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.

`snapshots.schedule`

Schedule for automatic volume snapshots

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

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

`volatile.uuid`

The volume’s UUID

Key: | `volatile.uuid`
Type: | string
Default: | random UUID

### 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