ZFS - zfs
Description: ZFS combines both physical volume management and a file system. A ZFS installation can span across a series of storage devices and is very scalable, allowing you to add disks to expand the availabl...

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# ZFS - `zfs`

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ZFS combines both physical volume management and a file system. A ZFS installation can span across a series of storage devices and is very scalable, allowing you to add disks to expand the available space in the storage pool immediately.

ZFS is a block-based file system that protects against data corruption by using checksums to verify, confirm and correct every operation. To run at a sufficient speed, this mechanism requires a powerful environment with a lot of RAM.

In addition, ZFS offers snapshots and replication, RAID management, copy-on-write clones, compression and other features.

To use ZFS, make sure you have `zfsutils-linux` installed on your machine.

## Terminology

ZFS creates logical units based on physical storage devices. These logical units are called ZFS pools or zpools. Each zpool is then divided into a number of datasets. These datasets can be of different types:

- A ZFS filesystem can be seen as a partition or a mounted file system.

- A ZFS volume represents a block device.

- A ZFS snapshot captures a specific state of either a ZFS filesystem or a ZFS volume. ZFS snapshots are read-only.

- A ZFS clone is a writable copy of a ZFS snapshot.

## `zfs` driver in LXD

The `zfs` driver in LXD uses ZFS filesystems and ZFS volumes for images and custom storage volumes, and ZFS snapshots and clones to create instances from images and for instance and custom volume snapshots. By default, LXD enables compression when creating a ZFS pool.

LXD assumes that it has full control over the ZFS pool and dataset. Therefore, you should never maintain any datasets or file system entities that are not owned by LXD in a ZFS pool or dataset, because LXD might delete them.

Due to the way copy-on-write works in ZFS, parent ZFS filesystems can’t be removed until all children are gone. As a result, LXD automatically renames any objects that are removed but still referenced. Such objects are kept at a random `deleted/` path until all references are gone and the object can safely be removed. Note that this method might have ramifications for restoring snapshots. See Limitations below.

LXD automatically enables trimming support on all newly created pools on ZFS 0.8 or later. This increases the lifetime of SSDs by allowing better block re-use by the controller, and it also allows to free space on the root file system when using a loop-backed ZFS pool. If you are running a ZFS version earlier than 0.8 and want to enable trimming, upgrade to at least version 0.8. Then use the following commands to make sure that trimming is automatically enabled for the ZFS pool in the future and trim all currently unused space:

    zpool upgrade ZPOOL-NAME
    zpool set autotrim=on ZPOOL-NAME
    zpool trim ZPOOL-NAME

### Limitations

The `zfs` driver has the following limitations:

Restoring from older snapshots

  ZFS doesn’t support restoring from snapshots other than the latest one. You can, however, create new instances from older snapshots. This method makes it possible to confirm whether a specific snapshot contains what you need. After determining the correct snapshot, you can remove the newer snapshots so that the snapshot you need is the latest one and you can restore it.

  Alternatively, you can configure LXD to automatically discard the newer snapshots during restore. To do so, set the `zfs.remove_snapshots` configuration for the volume (or the corresponding `volume.zfs.remove_snapshots` configuration on the storage pool for all volumes in the pool).

  Note, however, that if `zfs.clone_copy` is set to `true`, instance copies use ZFS snapshots too. In that case, you cannot restore an instance to a snapshot taken before the last copy without having to also delete all its descendants. If this is not an option, you can copy the wanted snapshot into a new instance and then delete the old instance. You will, however, lose any other snapshots the instance might have had.

Observing I/O quotas

  I/O quotas are unlikely to affect ZFS filesystems very much. That’s because ZFS is a port of a Solaris module (using SPL) and not a native Linux file system using the Linux VFS API, which is where I/O limits are applied.

Feature support in ZFS

  Some features, like the use of idmaps or delegation of a ZFS dataset, require ZFS 2.2 or higher and are therefore not widely available yet.

### Quotas

ZFS provides two different quota properties: `quota` and `refquota`. `quota` restricts the total size of a dataset, including its snapshots and clones. `refquota` restricts only the size of the data in the dataset, not its snapshots and clones.

By default, LXD uses the `quota` property when you set up a size/quota for your storage volume. If you want to use the `refquota` property instead, set the `zfs.use_refquota` configuration for the volume (or the corresponding `volume.zfs.use_refquota` configuration on the storage pool for all volumes in the pool).

You can also set the `zfs.reserve_space` (or `volume.zfs.reserve_space`) configuration to use ZFS `reservation` or `refreservation` along with `quota` or `refquota`.

## Configuration options

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

### Storage pool configuration

`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)
Scope: | local

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 ZFS dataset/pool

Key: | `source`
Type: | string
Scope: | local

`source.wipe`

Whether to wipe the block device before creating the pool

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

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

`zfs.clone_copy`

Whether to use ZFS lightweight clones

Key: | `zfs.clone_copy`
Type: | string
Default: | `true`
Scope: | global

Set this option to `true` or `false` to enable or disable using ZFS lightweight clones rather than full dataset copies. Set the option to `rebase` to copy based on the initial image.

`zfs.export`

Whether to export the zpool when an unmount is being performed

Key: | `zfs.export`
Type: | bool
Default: | `true`
Scope: | global

`zfs.pool_name`

Name of the zpool

Key: | `zfs.pool_name`
Type: | string
Default: | name of the pool
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

`block.filesystem`

File system of the storage volume

Key: | `block.filesystem`
Type: | string
Default: | same as `volume.block.filesystem`
Condition: | block-based volume with content type `filesystem` (`zfs.block_mode` enabled)
Scope: | global

Valid options are: `btrfs`, `ext4`, `xfs` If not set, `ext4` is assumed.

`block.mount_options`

Mount options for block-backed file system volumes

Key: | `block.mount_options`
Type: | string
Default: | same as `volume.block.mount_options`
Condition: | block-based volume with content type `filesystem` (`zfs.block_mode` enabled)
Scope: | global

`security.shared`

Enable volume sharing

Key: | `security.shared`
Type: | bool
Default: | same as `volume.security.shared` or `false`
Condition: | virtual-machine or custom block volume
Scope: | global

Enabling this option allows sharing the volume across multiple instances despite the possibility of data loss.

`security.shifted`

Enable ID shifting overlay

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

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
Scope: | global

`size`

Size/quota of the storage volume

Key: | `size`
Type: | string
Default: | same as `volume.size`
Condition: | appropriate driver
Scope: | global

`snapshots.expiry`

When snapshots are to be deleted

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

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
Scope: | global

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
Scope: | global

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.idmap.last`

JSON-serialized UID/GID map that has been applied to the volume

Key: | `volatile.idmap.last`
Type: | string
Condition: | filesystem

`volatile.idmap.next`

JSON-serialized UID/GID map that has been applied to the volume

Key: | `volatile.idmap.next`
Type: | string
Condition: | filesystem

`volatile.uuid`

The volume’s UUID

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

`zfs.block_mode`

Whether to use a formatted `zvol` rather than a dataset

Key: | `zfs.block_mode`
Type: | bool
Default: | same as `volume.zfs.block_mode`
Scope: | global

`zfs.block_mode` can be set only for custom storage volumes. To enable ZFS block mode for all storage volumes in the pool, including instance volumes, use `volume.zfs.block_mode`.

`zfs.blocksize`

Size of the ZFS block

Key: | `zfs.blocksize`
Type: | string
Default: | same as `volume.zfs.blocksize`
Scope: | global

The size must be between 512 bytes and 16 MiB and must be a power of 2. For a block volume, a maximum value of 128 KiB will be used even if a higher value is set.

Depending on the value of `zfs.block_mode`, the specified size is used to set either `volblocksize` or `recordsize` in ZFS.

`zfs.delegate`

Whether to delegate the ZFS dataset

Key: | `zfs.delegate`
Type: | bool
Default: | same as `volume.zfs.delegate`
Condition: | ZFS 2.2 or higher
Scope: | global

This option controls whether to delegate the ZFS dataset and anything underneath it to the container or containers that use it. When used in conjunction with `security.nesting`, this allows using the `zfs` command in the container.

`zfs.remove_snapshots`

Remove snapshots as needed

Key: | `zfs.remove_snapshots`
Type: | bool
Default: | same as `volume.zfs.remove_snapshots` or `false`
Scope: | global

`zfs.reserve_space`

Use `reservation`/`refreservation` along with `quota`/`refquota`

Key: | `zfs.reserve_space`
Type: | bool
Default: | same as `volume.zfs.reserve_space` or `false`
Scope: | global

`zfs.use_refquota`

Use `refquota` instead of `quota` for space

Key: | `zfs.use_refquota`
Type: | bool
Default: | same as `volume.zfs.use_refquota` or `false`
Scope: | global

### 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
Scope: | local