[PVE-User] Proxmox with ceph storage VM performance strangeness

Rainer Krienke krienke at uni-koblenz.de
Tue Apr 14 20:15:15 CEST 2020

Am 14.04.20 um 18:09 schrieb Alwin Antreich:

>> In a VM I also tried to read its own striped LV device: dd
>> if=/dev/vg/testlv  of=/dev/null bs=1024k status=progress (after clearing
>> the VMs cache). /dev/vg/testlv is a striped LV (on 4 disks) with xfs on
>> it  on which I tested the speed using bonnie++ before.
>> This dd also did not go beyond about 100MB/sec, whereas the rados bench
>> promises much more.
> Do you have a VM without stripped volumes? I suppose there will be two
> requests, for each half of the data. That could slow down the read as> well.

Yes the logical volume is striped using 4 physical volumes (RBDs). But
since exactly this setup helped to boost up writing (more paralellism)
it should do exactly the same since blocks can be read from more
separate rbd devices and thus more disks in general.

I also tested a VM with just a single rbd used for the VMs disk and
there the effect ist quite the same.

> And you can disable the cache to verify that cache misses don't impact
> the performance.

I tried and disabled the writeback cache but the effect was only minimal.

Have a nice day
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

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