LVM - lvm

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LVM is a storage management framework rather than a file system. It is used to manage physical storage devices, allowing you to create a number of logical storage volumes that use and virtualize the underlying physical storage devices.

Note that it is possible to over-commit the physical storage in the process, to allow flexibility for scenarios where not all available storage is in use at the same time.

To use LVM, make sure you have lvm2 installed on your machine.

Terminology

LVM can combine several physical storage devices into a volume group. You can then allocate logical volumes of different types from this volume group.

One supported volume type is a thin pool, which allows over-committing the resources by creating thinly provisioned volumes whose total allowed maximum size is larger than the available physical storage. Another type is a volume snapshot, which captures a specific state of a logical volume.

lvm driver in LXD

The lvm driver in LXD uses logical volumes for images, and volume snapshots for instances and snapshots.

LXD assumes that it has full control over the volume group. Therefore, you should not maintain any file system entities that are not owned by LXD in an LVM volume group, because LXD might delete them. However, if you need to reuse an existing volume group (for example, because your setup has only one volume group), you can do so by setting the lvm.vg.force_reuse configuration.

By default, LVM storage pools use an LVM thin pool and create logical volumes for all LXD storage entities (images, instances and custom volumes) in there. This behavior can be changed by setting lvm.use_thinpool to false when you create the pool. In this case, LXD uses “normal” logical volumes for all storage entities that are not snapshots. Note that this entails serious performance and space reductions for the lvm driver (close to the dir driver both in speed and storage usage). The reason for this is that most storage operations must fall back to using rsync, because logical volumes that are not thin pools do not support snapshots of snapshots. In addition, non-thin snapshots take up much more storage space than thin snapshots, because they must reserve space for their maximum size at creation time. Therefore, this option should only be chosen if the use case requires it.

For environments with a high instance turnover (for example, continuous integration) you should tweak the backup retain_min and retain_days settings in /etc/lvm/lvm.conf to avoid slowdowns when interacting with LXD.

Configuration options

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

Storage pool configuration

Key	Type	Default	Description
lvm.thinpool_name	string	LXDThinPool	Thin pool where volumes are created
lvm.thinpool_metadata_size	string	0 (auto)	The size of the thin pool metadata volume (the default is to let LVM calculate an appropriate size)
lvm.use_thinpool	bool	true	Whether the storage pool uses a thin pool for logical volumes
lvm.vg.force_reuse	bool	false	Force using an existing non-empty volume group
lvm.vg_name	string	name of the pool	Name of the volume group to create
rsync.bwlimit	string	0 (no limit)	The upper limit to be placed on the socket I/O when rsync must be used to transfer storage entities
rsync.compression	bool	true	Whether to use compression while migrating storage pools
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 an existing block device, loop file or LVM volume group
source.wipe	bool	false	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

Key	Type	Condition	Default	Description
block.filesystem	string	block based driver	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 driver	same as volume.block.mount_options	Mount options for block devices
lvm.stripes	string	LVM driver	same as volume.lvm.stripes	Number of stripes to use for new volumes (or thin pool volume)
lvm.stripes.size	string	LVM driver	same as volume.lvm.stripes.size	Size of stripes to use (at least 4096 bytes and multiple of 512 bytes)
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>), a comma-separated list of schedule aliases (@hourly, @daily, @midnight, @weekly, @monthly, @annually, @yearly), or empty to disable automatic snapshots (the default)