[PVE-User] High write iops rate from idle VMs running on a PVE cluster with ceph storage

Rainer Krienke krienke at uni-koblenz.de
Mon Jul 12 08:53:03 CEST 2021


Hello,

I run a 5 node PVE cluster with pve-manager/6.4-8/185e14db (running 
kernel: 5.4.119-1-pve). The storage backend is a HDD based "external" 
ceph cluster running Ceph 14.2.16 with 144 OSDs on 9 hosts. Currently 
there are about 70 VMs running on this PVE cluster, all Linux (Ubuntu, 
SLES).

The problem I have is that writing on VMS has become slower and slower 
over time and eg running linux updates (eg apt upgrade) on the VMS takes 
longer and longer. The reason seams to be a steadily rising write IOPs 
rate on the storage side. Of course over time the number of VNMs also 
increased up to the current number causing higher numbers.

Over the week day I can see rates on the ceph side of up to 1000 
IOPS/sec writing and about 300 IOPS/sec reading. The really stange thing 
is however that even at weekends where the services the VMs offer are 
hardly used at all, there is still a quite high write IOPS rate of about 
400/sec whereas the read rate is only about 50 IOPS/sec then. The Bytes 
read/written are minimal at this time with only about 100KBytes read/sec 
and  about 5MBytes write/sec.

So what I am looking for is by what the "always there" write IOPS-Rate 
of about 400 could be caused.  My guess is that this could be caused by 
file time (mtime,ctime,atime) write updates to the VMs filesystems.  If 
this was true then using lazytime in /etc/fstab on all VMs could help to 
avoid this behaviour.

But on the other hand all VMs use the (safe) "Writeback"-cache setting. 
So shouldn't this cache mode also cache writes caused by updates for 
file times?

If yes, than I have to look for other reasons for my write IOPS problem 
allthough I have no idea about this at the moment.  Any  suggestions?

Thanks
Rainer
-- 
Rainer Krienke, Uni Koblenz, Rechenzentrum, A22, Universitaetsstrasse  1
56070 Koblenz, Web: http://www.uni-koblenz.de/~krienke, Tel: +49261287 1312
PGP: http://www.uni-koblenz.de/~krienke/mypgp.html,     Fax: +49261287 
1001312



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