[pve-devel] qcow2 internal snapshot: 4k write speed reduce to 100~200 iops on ssd ?

[Fiona Ebner]

Hi,

Am 30.08.24 um 18:01 schrieb DERUMIER, Alexandre via pve-devel:
> Hi,
> 
> I was doing tests with gfs2 && ocfs2,
> 
> and I have notice 4k randwrite iops going from 20000 iops to
> 100~200iops when a snapshot is present.
> 
> I thinked it was related to gfs2 && ocfs2 allocation, 
> but I can reproduce too with a simple qcow2 file on
> a local ssd drive.
> 

Is the performance drop also this big with the local storage?

> is it expected ?
> 
> (I don't have use qcow2 snapshot since 10year, so I really don't 
>  remember performance).
> 

A performance drop is of course expected, because AFAIU it needs to do
COW for the sectors that the snapshot references, but the drop does
sound rather dramatic in your case. The performance should be better
again once the COW is done, e.g. when running another test afterwards. I
quickly tested in a nested VM with qcow2 disk on a directory storage and
--numjobs=4:

ran test, got ~33k IOPS
made a snapshot
ran test, got ~20k IOPS (it starts out very slow and then recovers)
ran test, got ~33k IOPS again

Best Regards,
Fiona

> 
> Using external snapshot file, I'm around 10000iops.
> 
> test inside the vm:
> 
> fio --filename=/dev/sdX --ioengine=libaio --direct 1 --bs=4k  --
> rw=randwrite --numjobs=64
> 
>