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

Daniel Berteaud daniel at firewall-services.com
Thu Jul 23 08:53:01 CEST 2020

----- Le 23 Juil 20, à 8:43, Fabian Grünbichler f.gruenbichler at proxmox.com a écrit :

> 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

I'm running pve-qemu-kvm 5.0.0-11, and all the implied VM have been either (cold) rebooted, or migrated.

> 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.

Yes, I'd expect some amplification, but not that much. For my Zabbix server, it's nearly canceling all the benefit of using a dirty bitmap.
One thing I've noted, is that I get expected values at least for one guest, running PfSense (where I get ~150MB of dirty blocks each days). Most of my other VM are Linux, I'll check if it could be related to the atime update or something


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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 
Matrix: @dani:fws.fr 
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