Re: [PVE-User] Proxmox Backup Server (beta)

[Thomas Lamprecht <t.lamprecht@proxmox.com>]

On 18.07.20 16:59, Tom Weber wrote:
> Am Freitag, den 17.07.2020, 19:43 +0200 schrieb Thomas Lamprecht:
>> If a backup fails, or the last backup index we get doesn't matches
>> the
>> checksum we cache in the VM QEMU process we drop the bitmap and do
>> read
>> everything (it's still send incremental from the index we got  now),
>> and
>> setup a new bitmap from that point.
> 
> ah, I think I start to understand (read a bit about the qemu side too
> now) :)
> 
> So you keep some checksum/signature of a successfull backup run with
> the one (non-persistant) dirty bitmap in qemu.
> The next backup run can check this and only makes use of the bitmap if
> it matches else it will fall back to reading and comparing all qemu
> blocks against the ones in the backup - saving only the changed ones? 

Exactly.

> First of all, that noted backup scenario was not designed for a
> blocklevel incremental backup like pbs is meant. I don't know yet if
> I'd do it like this for pbs. But it probably helps to understand why it
> raised the above question.
> 
> If the same "area" of data changes everyday, say 1GB, and I do
> incremental backups and have like 10GB of space for that on 2
> independent Servers.
> Doing that incremental Backup odd/even to those 2 Backupservers, I end
> up with 20 days of history whereas with 2 syncronized Backupservers
> only 10 days of history are possible (one could also translate this in
> doubled backup space ;) ).

Yeah if only the same disk blocks are touched the math would work out.
But you've doubled the risk of loosing the most recent backup, that's
the price.

But you do you, I'd honestly just check it out and test around a bit to
see how it really behaves for your use case and setup behavior and
limitations.

cheers,
Thomas