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

Roland devzero at web.de
Thu Jul 23 23:59:31 CEST 2020


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....

regards
roland


Am 23.07.20 um 08:53 schrieb Daniel Berteaud:
> ----- 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
>
> Cheers,
> Daniel
>



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