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 ** Summary changed: - win2k3 guest has blank screen and high cpu on first boot + qcow2 unusably slow when migrating from 8.04LTS to 10.04LTS -- You received this bug notification because you are a member of Ubuntu Server Team, which is subscribed to qemu-kvm in ubuntu. https://bugs.launchpad.net/bugs/671163 Title: qcow2 unusably slow when migrating from 8.04LTS to 10.04LTS -- Ubuntu-server-bugs mailing list Ubuntu-server-bugs@lists.ubuntu.com Modify settings or unsubscribe at: https://lists.ubuntu.com/mailman/listinfo/ubuntu-server-bugs
