[PVE-User] ZFS - question

[Thomas Lamprecht]

Hi,

Am 8/13/19 um 10:37 AM schrieb lord_Niedzwiedz:
> 
> I run a "Stop" backup on proxmox, it shuts down the machine.
> Starts making a copy.
> But it immediately turns it on ("restarts only", doesn't stop for the duration of the copy !! - why ??).
> "resuming VM again after 21 seconds" ?? !! why like this ?
> 
> Is it better to make a Snapshot copy ?? (its qestion number three ;) )

First, can you please post such threads as new ones, not as reply-to to
an existing one ("Proxmox VE 6.0 released!" in this case). That'd be
great, as it keeps inboxes a bit cleaner and it just make sense to
separate different, especially new, topics and questions by thread.
Thanks!

That said, "Stop" mode backups shut the machine down first, to bring it
in a consistent state. Then, the machine is started again in a "paused"
state - here we save the state of the consistent disk blocks, and then
resume the VM while we start backing up the data. If the VM writes to
some data we did not yet backed up we can detect that and save the changes
somewhere else (simplified said) while we backup the consistent data.

This mode guarantees that the backup was made from a consistent state,
but also that the VM can resume its operation relatively fast again.


If your underlying storage supports snapshots in a performant way -
like, among others, ZFS, LVM-Thin or a qcow2 backed disk do - you can
use that mode too. But, it does not has the same consistency guarantees
the stop mode has, there is some small chance to inconsistency - it
depends totally on the applications and workload running inside the VM.
Mostly the inconsistency risk here is the same as with a powerloss -
if the applications running inside can cope with that well you won't
have issues.

You can use snapshot as a full replacement for stop mode in combination
with the guest agent, though, as then we issue a "guest-fsfreeze-freeze"
before doing the snapshot - that should ensure consistency.

See:
https://pve.proxmox.com/pve-docs/chapter-vzdump.html#_backup_modes
for some documentation regarding this.

cheers,
Thomas