[PVE-User] PBS : is dirty-bitmap really accurate ?
f.gruenbichler at proxmox.com
Thu Jul 23 08:43:06 CEST 2020
On July 22, 2020 6:40 pm, Daniel Berteaud wrote:
> I've started playing with PBS on some VM. So far, it's looking really promizing.
> There's one strange thing though : the percent of the dirty data. For example, I backup one VM every 2 or 3 days. It's a moderately busy server, mainly serving a MariaDB database (zabbix server + mariadb). On each backup, I get similar dirty values :
> INFO: using fast incremental mode (dirty-bitmap), 492.7 GiB dirty of 590.0 GiB total
> While I'm sur not even 10% of this has really been written.
> Get more or less the same problem on other VM. One which I know just sleep all day (my personnal OnlyOffice document server), and which I backup daily, and get values like :
> INFO: using fast incremental mode (dirty-bitmap), 5.0 GiB dirty of 10.0 GiB total
> Or another small one (personnal samba DC controler) :
> INFO: using fast incremental mode (dirty-bitmap), 13.0 GiB dirty of 20.0 GiB total
> The only write activity for those 2 are just a few KB or maybe MB of log lines. Respectivly 5 and 13GB of dirty blocks seems unreal.
> Am I the only one seeing this ? Could the dirty-bitmap mark dirty blocks without write activity somehow ?
possibly you haven't upgraded to pve-qemu-kvm 5.0-11 (or your VM hasn't
been restarted yet since the upgrade):
the bitmap has a granularity of 4MB, so depending on the activity inside
you can see quite a bit of amplification. also writing and then
zeroing/reverting again to the old content would leave a mark in the
bitmap without permanently changing the contents.
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