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

Daniel Berteaud daniel at firewall-services.com
Fri Jul 24 09:02:30 CEST 2020

----- Le 23 Juil 20, à 23:59, Roland devzero at web.de a écrit :

> i just had a look on that, too - i was backing up a VM i backed up this
> afternoon,
> and for my curiosity 1GB out of 15GB was marked dirty.
> that looks a quite much for me for a mostly idle system, because there
> was definitely only a little bit of change on the system within logfiles
> in /var/log
> so i wonder what marked all that blocks dirty....
> i'm also suspecting atime changes...will keep an eye on that....

It was my daily fstrim in my case. Each time you trim, it'll dirty all the blocks corresponding to unused space.
I've switched this to a weekly job so bacups can run most of the time efficiently. Since then, dirty blocks went from ~15GB per VM on average to something betwwen 800MB and 4GB, which is much closer to what I expect, considering the 4MB granularity of the bitmap.

I'll check on my biggest VM (Zabbix server) but I expect similar results

Thanks Fabian and Jorge for pointing this out


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Daniel Berteaud 
FIREWALL-SERVICES SAS, La sécurité des réseaux 
Société de Services en Logiciels Libres 
Tél : +33.5 56 64 15 32 
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