Ubuntu
qemu-kvm package

  1. Bug #671163
  2. Comment #16

Comment 16 for bug 671163

Revision history for this message
Steven Wagner (stevenwagner) wrote : Re: win2k3 guest has blank screen and high cpu on first boot

It appears that writeback caching was turned off in qemu for qcow2 in June 2009 because qcow2 was "fast". Then a new patch was applied in July 2010 that made qcow2 slow again! I was able to solve my problems by turning on writeback cache.

<driver name='qemu' type='qcow2' cache='writeback'/>

However, this creates a huge migration issue for anyone upgrading from Ubuntu Server 8.04 LTS to 10.04 LTS. If they use qcow2 files with qemu-kvm, their virtualization guests will fail and start having disk IO failures if they are not aware that they now need to turn on writeback caching manually.

see links about qcow2 changes:

https://lists.ubuntu.com/archives/ubuntu-server/2010-March/003910.html

http://us.generation-nt.com/answer/bug-594069-qemu-kvm-really-poor-write-performance-qcow2-hdd-image-help-199995311.html