Storage drivers

LXD supports the following storage drivers for storing images, instances and custom volumes:

See the corresponding pages for driver-specific information and configuration options.

Feature comparison

Where possible, LXD uses the advanced features of each storage system to optimize operations.

Feature

Directory

Btrfs

LVM

ZFS

Ceph RBD

CephFS

Ceph Object

Dell PowerFlex

Optimized image storage

❌

✅

✅

✅

✅

➖

➖

❌

Optimized instance creation

❌

✅

✅

✅

✅

➖

➖

❌

Optimized snapshot creation

❌

✅

✅

✅

✅

✅

➖

✅

Optimized image transfer

❌

✅

❌

✅

✅

➖

➖

❌

Optimized backup (import/export)

❌

✅

❌

✅

❌

➖

➖

❌

Optimized volume transfer

❌

✅

❌

✅

✅[1]

➖

➖

❌

Optimized volume refresh

❌

✅

✅[2]

✅

✅[3]

➖

➖

❌

Copy on write

❌

✅

✅

✅

✅

✅

➖

✅

Block based

❌

❌

✅

❌

✅

❌

➖

✅

Instant cloning

❌

✅

✅

✅

✅

✅

➖

❌

Storage driver usable inside a container

✅

✅

❌

✅[4]

❌

➖

➖

❌

Restore from older snapshots (not latest)

✅

✅

✅

❌

✅

✅

➖

✅

Storage quotas

✅[5]

✅

✅

✅

✅

✅

✅

✅

Available on lxd init

✅

✅

✅

✅

✅

❌

❌

❌

Object storage

✅

✅

✅

✅

❌

❌

✅

❌

Optimized image storage

Most of the storage drivers have some kind of optimized image storage format. To make instance creation near instantaneous, LXD clones a pre-made image volume when creating an instance rather than unpacking the image tarball from scratch.

To prevent preparing such a volume on a storage pool that might never be used with that image, the volume is generated on demand. Therefore, the first instance takes longer to create than subsequent ones.

Optimized volume transfer

Btrfs, ZFS and Ceph RBD have an internal send/receive mechanism that allows for optimized volume transfer.

LXD uses this optimized transfer when transferring instances and snapshots between storage pools that use the same storage driver, if the storage driver supports optimized transfer and the optimized transfer is actually quicker. Otherwise, LXD uses rsync to transfer container and file system volumes, or raw block transfer to transfer virtual machine and custom block volumes.

The optimized transfer uses the underlying storage driver’s native functionality for transferring data, which is usually faster than using rsync or raw block transfer.

Optimized volume refresh

The full potential of the optimized transfer becomes apparent when refreshing a copy of an instance or custom volume that uses periodic snapshots. If the optimized transfer isn’t supported by the driver or its implementation of volume refresh, instead of the delta, the entire volume including its snapshot(s) will be copied using either rsync or raw block transfer. LXD will try to keep the overhead low by transferring only the volume itself or any snapshots that are missing on the target.

When optimized refresh is available for an instance or custom volume, LXD bases the refresh on the latest snapshot, which means:

  • When you take a first snapshot and refresh the copy, the transfer will take roughly the same time as a full copy. LXD transfers the new snapshot and the difference between the snapshot and the main volume.

  • For subsequent snapshots, the transfer is considerably faster. LXD does not transfer the full new snapshot, but only the difference between the new snapshot and the latest snapshot that already exists on the target.

  • When refreshing without a new snapshot, LXD transfers only the differences between the main volume and the latest snapshot on the target. This transfer is usually faster than using rsync (as long as the latest snapshot is not too outdated).

On the other hand, refreshing copies of instances without snapshots (either because the instance doesn’t have any snapshots or because the refresh uses the --instance-only flag) would actually be slower than using rsync or raw block transfer. In such cases, the optimized transfer would transfer the difference between the (non-existent) latest snapshot and the main volume, thus the full volume. Therefore, LXD uses rsync or raw block transfer instead of the optimized transfer for refreshes without snapshots.

Security considerations

Currently, the Linux kernel might silently ignore mount options and not apply them when a block-based file system (for example, ext4) is already mounted with different mount options. This means when dedicated disk devices are shared between different storage pools with different mount options set, the second mount might not have the expected mount options. This becomes security relevant when, for example, one storage pool is supposed to provide acl support and the second one is supposed to not provide acl support.

For this reason, it is currently recommended to either have dedicated disk devices per storage pool or to ensure that all storage pools that share the same dedicated disk device use the same mount options.