ZFS - zfs¶
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
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
LXD assumes that it has full control over the ZFS pool and
Due to the way copy-on-write works in ZFS, parent 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_snapshotsconfiguration for the volume (or the correspondingvolume.zfs.remove_snapshotsconfiguration on the storage pool for all volumes in the pool).Note, however, that if
zfs.clone_copyis set totrue, 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 refquota restricts only the size of the data in the
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¶
| 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.
| Key: | source |
| Type: | string |
| Scope: | local |
| Key: | source.wipe |
| Type: | bool |
| Default: |
|
| Scope: | local |
Set this option to true to wipe the block device specified in source
prior to creating the storage pool.
| Key: | zfs.clone_copy |
| Type: | string |
| Default: |
|
| 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.
| Key: | zfs.export |
| Type: | bool |
| Default: |
|
| Scope: | global |
| 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¶
| Key: | block.filesystem |
| Type: | string |
| Default: | same as |
| Condition: | block-based volume with content type |
| Scope: | global |
Valid options are: btrfs, ext4, xfs
If not set, ext4 is assumed.
| Key: | block.mount_options |
| Type: | string |
| Default: | same as |
| Condition: | block-based volume with content type |
| Scope: | global |
| Key: | security.shifted |
| Type: | bool |
| Default: | same as |
| Condition: | custom volume |
| Scope: | global |
Enabling this option allows attaching the volume to multiple isolated instances.
| Key: | security.unmapped |
| Type: | bool |
| Default: | same as |
| Condition: | custom volume |
| Scope: | global |
| Key: | size |
| Type: | string |
| Default: | same as |
| Condition: | appropriate driver |
| Scope: | local |
| Key: | snapshots.expiry |
| Type: | string |
| Default: | same as |
| Condition: | custom volume |
| Scope: | global |
Specify an expression like 1M 2H 3d 4w 5m 6y.
| Key: | snapshots.pattern |
| Type: | string |
| Default: | same as |
| 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.
| Key: | snapshots.schedule |
| Type: | string |
| Default: | same as |
| 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).
| Key: | zfs.block_mode |
| Type: | bool |
| Default: | same as |
| 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.
| Key: | zfs.blocksize |
| Type: | string |
| Default: | same as |
| 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.
| Key: | zfs.delegate |
| Type: | bool |
| Default: | same as |
| 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.
| Key: | zfs.remove_snapshots |
| Type: | bool |
| Default: | same as |
| Scope: | global |
| Key: | zfs.reserve_space |
| Type: | bool |
| Default: | same as |
| 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.