[PVE-User] pbs incremental backups

Andreas Heinlein aheinlein at gmx.com
Tue Mar 2 11:37:36 CET 2021


this is probably a bit difficult to understand at first, but I will try 
to explain.

Each backup is a full backup, even though only the differences to the 
last backup are actually backed up. The remaining unchanged block are 
just being referenced from the last backup. When you delete a previous 
backup, only those blocks which are not referenced by another backup 
will actually be deleted.

So each backup is independent on its own, and you can delete all 
previous backups without losing any data.

Hope this clears it up a bit.


Am 02.03.2021 um 11:22 schrieb mj:
> Hi,
> Testing PBS backups taken from PVE VMs on ceph rbd now. Very nice, 
> very quick, very cool. :-)
> We have a question. Something we wonder about.
> In our current backup software, we make weekly full_system backups, 
> and daily incremental_system backups, each incremental based on the 
> same full_system backup. So: each daily incremental backup becomes 
> bigger, until the weekend. Then we make a new full_system backup to 
> base the next set of incrementals on.
> In PBS I cannot specify if a backup is full or incremental, we assume 
> this means that automatically the first backup is a full_system 
> backup, and subsequent backups are incremental. The PBS backup logs 
> confirm this assumption, saying: "scsi0: dirty-bitmap status: created 
> new" vs "scsi0: dirty-bitmap status: OK (7.3 GiB of 501.0 GiB dirty)"
> And now the question: At what point in time is a new full_system 
> backup created, to rebase incremental backups on?
> Or is each incremental backup based on the previous incremental? And 
> if that is the case, how will we ever be able to delete one of the 
> in-between incrementals, because that would then break to whole chain 
> of incremental_backup-based-on-incremental_backup...?
> We have read the page 
> https://qemu.readthedocs.io/en/latest/interop/bitmaps.html but it does 
> not seem to answer this.
> Anyone care to share some insight on this logic and how PBS works?
> MJ 

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