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

Ronny Aasen ronny+pve-user at aasen.cx
Fri Jul 24 09:54:13 CEST 2020


On 24.07.2020 09:02, Daniel Berteaud wrote:
> ----- 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
> 
> Cheers
> Daniel
> 


would mounting the disk with discard help on this ? where it only trims 
blocks that are actually discarded ? instead of touching the whole disk 
with fstrim ?


Ronny




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