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 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 Incus

The zfs driver in Incus 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, Incus enables compression when creating a ZFS pool.

Incus 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 Incus in a ZFS pool or dataset, because Incus 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, Incus 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.

Incus 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 Incus 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, Incus uses the quota property when you set up a 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.use_reserve_space (or volume.zfs.use_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	Type	Default	Description
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, can be increased to grow storage pool)
source	string	-	Path to existing block device(s), loop file or ZFS dataset/pool. Multiple block devices should be separated by ,. When listing block devices, you can also prefix them with vdev type. To specify a vdev type, use an = sign between the vdev type and the block devices (e.g., mirror=/dev/sda,/dev/sdb). Only stripe, mirror, raidz1 and raidz2 vdev types are supported.
source.wipe	bool	false	Wipe the block device specified in source prior to creating the storage pool
zfs.clone_copy	string	true	Whether to use ZFS lightweight clones rather than full dataset copies (Boolean), or rebase to copy based on the initial image
zfs.export	bool	true	Disable zpool export while unmount performed
zfs.pool_name	string	name of the pool	Name of the zpool

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
block.filesystem	string	block-based volume with content type filesystem (zfs.block_mode enabled)	same as volume.block.filesystem	File system of the storage volume: btrfs, ext4 or xfs (ext4 if not set)
block.mount_options	string	block-based volume with content type filesystem (zfs.block_mode enabled)	same as volume.block.mount_options	Mount options for block-backed file system volumes
security.shared	bool	custom block volume	same as volume.security.shared or false	Enable sharing the volume across multiple instances
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		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) [1]
snapshots.schedule	string	custom volume	same as snapshots.schedule	Cron expression (<minute> <hour> <dom> <month> <dow>), a comma-separated list of schedule aliases (@hourly, @daily, @midnight, @weekly, @monthly, @annually, @yearly), or empty to disable automatic snapshots (the default)
zfs.blocksize	string		same as volume.zfs.blocksize	Size of the ZFS block in range from 512 to 16 MiB (must be power of 2) - for block volume, a maximum value of 128 KiB will be used even if a higher value is set
zfs.block_mode	bool		same as volume.zfs.block_mode	Whether to use a formatted zvol rather than a dataset (zfs.block_mode can be set only for custom storage volumes; use volume.zfs.block_mode to enable ZFS block mode for all storage volumes in the pool, including instance volumes)
zfs.delegate	bool	ZFS 2.2 or higher	same as volume.zfs.delegate	Controls whether to delegate the ZFS dataset and anything underneath it to the container(s) using it. Allows the use of the zfs command in the container.
zfs.remove_snapshots	bool		same as volume.zfs.remove_snapshots or false	Remove snapshots as needed
zfs.use_refquota	bool		same as volume.zfs.use_refquota or false	Use refquota instead of quota for space
zfs.reserve_space	bool		same as volume.zfs.reserve_space or false	Use reservation/refreservation along with quota/refquota

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.

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

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[1]

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.