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vzdump: add section about backup fleecing

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Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
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Fiona Ebner 2024-04-11 11:29:43 +02:00
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@ -136,6 +136,44 @@ not included in backups. For volume mount points you can set the *Backup* option
to include the mount point in the backup. Device and bind mounts are never
backed up as their content is managed outside the {pve} storage library.
VM Backup Fleecing
~~~~~~~~~~~~~~~~~~
WARNING: Backup fleecing is still being worked on (also in upstream QEMU) and is
currently only a technology preview.
When a backup for a VM is started, QEMU will install a "copy-before-write"
filter in its block layer. This filter ensures that upon new guest writes, old
data still needed for the backup is sent to the backup target first. The guest
write blocks until this operation is finished so guest IO to not-yet-backed-up
sectors will be limited by the speed of the backup target.
With backup fleecing, such old data is cached in a fleecing image rather than
sent directly to the backup target. This can help guest IO performance and even
prevent hangs in certain scenarios, at the cost of requiring more storage space.
Use e.g. `vzdump 123 --fleecing enabled=1,storage=local-lvm` to enable backup
fleecing, with fleecing images created on the storage `local-lvm`.
The fleecing storage should be a fast local storage, with thin provisioning and
discard support. Examples are LVM-thin, RBD, ZFS with `sparse 1` in the storage
configuration, many file-based storages. Ideally, the fleecing storage is a
dedicated storage, so it running full will not affect other guests and just fail
the backup. Parts of the fleecing image that have been backed up will be
discarded to try and keep the space usage low.
For file-based storages that do not support discard (e.g. NFS before version
4.2), you should set `preallocation off` in the storage configuration. In
combination with `qcow2` (used automatically as the format for the fleecing
image when the storage supports it), this has the advantage that already
allocated parts of the image can be re-used later, which can still help save
quite a bit of space.
WARNING: On a storage that's not thinly provisioned, e.g. LVM or ZFS without the
`sparse` option, the full size of the original disk needs to be reserved for the
fleecing image up-front. On a thinly provisioned storage, the fleecing image can
grow to the same size as the original image only if the guest re-writes a whole
disk while the backup is busy with another disk.
Backup File Names
-----------------