[PVE-User] PBS : is dirty-bitmap really accurate ?

[Fabian Grünbichler]

On July 22, 2020 6:40 pm, Daniel Berteaud wrote:
> Hi 
> 
> 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):

 https://git.proxmox.com/?p=pve-qemu.git;a=commit;h=f257cc05f4fbf772cad3231021b3ce7587127a1b

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.