Storage: Restore VM filesystem size.state quota on backup restore #9628
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…ckup restore Use size.state (if set) or otherwise use default 100MiB setting if size is non-empty, to apply quota to VM's filesystem volume. Signed-off-by: Thomas Parrott <thomas.parrott@canonical.com>
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Use
size.state(if set) or otherwise use default 100MiB setting ifsizeis set, to apply quota to VM's filesystem volume.Warning:
Whilst this PR does fix a bug, it will introduce further issues with the BTRFS storage driver.
Due to the way that BTRFS accounts for subvolume quota by conflating both CoW snapshot writes and actual new usage in a subvolume, this will cause issues with VM snapshots.
This is because when applying a VM disk quota, the LXD BTRFS driver will set the subvolume quota to
<size.state>+<raw disk image size>, wheresize.statedefaults to 100MiB.When restoring a non-optimised VM backup, BTRFS will account (correctly) that 100% of the raw disk file has been written to.
But this will then mean that when creating a VM snapshot, that only up to 100MiB (or whatever
size.state) can be written to the VM's disk before the BTRFS quota is reached, causing I/O errors in the VM. This is because despite of settingnodatacowfor new VM disk files (https://github.com/lxc/lxd/pull/9627) the initial write to an extent after a snapshot has been taken will necessarily trigger a CoW (in order to keep track of the snapshot's contents). BTRFS extents can be up to 128MiB in size, and this will then cause both the old and new extents to be accounted toward the subvolume's quota usage.Previously this bug masked the issue by not enforcing a BTRFS quota for restored VM backups.
Related to #9124
Signed-off-by: Thomas Parrott thomas.parrott@canonical.com