qcow2 internal snapshot: 4k write speed reduce to 100~200 iops on ssd ?

[DERUMIER, Alexandre]

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 it expected ?

(I don't have use qcow2 snapshot since 10year, so I really don't 
 remember performance).


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