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Question regarding qemu-img convert when using LUNS with LVM

From: Thomas Schneider
Subject: Question regarding qemu-img convert when using LUNS with LVM
Date: Wed, 6 Jan 2021 15:32:54 +0000

Hello everyone,

When I use a simple qemu-img convert to move from a storage of my choice onto a multipath LUN with LVM I have massive performance problems for all VMs running on that LUN. It looks like the qemu process locks the whole LUN so that no disk writes are performed. I can see in iotop that nothing happens. As soon as the convert really reads and writes data this issue is resolved and everything is responsive.

Thats an example command I execute to reproduce this issue:

qemu-img convert -p -n -f raw -O raw /vz/images/999/vm-999-disk-1.raw /dev/nf-lvmstorage/vm-999-disk-0

Can anybody tell me what is causing this issue? The LUN is very fast and its connected with 10Gbit fiber. When the convert is done any other LUNs also located on the same storage run flawless. On the storage you can also not see any relevant high load. 

Thanks alot in advance!


Thomas Schneider
DevOps Engineer
NetFederation GmbH

. . Fon: +49 (0)2236/3936-708
E-Mail: Thomas.Schneider@net-federation.de - www.netfed.de
Sürther Hauptstraße 180 B - Köln 50999

NetFederation GmbH
Geschäftsführung: Christian Berens, Thorsten Greiten, Edin Rekic 
Amtsgericht Köln, HRB Nr. 32660

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