Hello, I'm currently testing some distribution with uptodate/new KVM : - Ubuntu Karmic 9.10 - Rhel 5.4 I'm using IoZone to test IO disk performance. For my test I have dedicated a LVM partition to the IoZone benchmark : iozone -a -U /mnt/bench/ -f /mnt/bench/test-file -R -b <file.xls> I have done multiple tests with 'cache=none', cache='writeback' and cache='writethrough'. Inside the VM I have tried, for the filesystem, the options data=ordered and data=writeback. All my tests have different results but at last, something is strange : in a VM, IO Disk performance are better than in the host system for little files and blocks and worse for big files and blocks. I can give all my iozone benchmark results if you'd like ;). Well I do some assumption : - KVM provide a cache for read/write operations - KVM tells to the guest that data are written, but in fact, they are not. Can someone explain me this behaviour ? Can I control it ? Can this lead to data corruption in case of hardware crash ? More informations ============================== Ubuntu 9.10 ----------------------------------- - host : Ubuntu 9.10 ; kernel 2.6.31 ; qemu-kvm 0.11 ; lvm dedicated partition in ext3 (data=ordered or writeback). - guest : Ubuntu 9.10 kernel 2.6.31 ; qemu-kvm 0.11 ; virtio disk; lvm dedicated partition in ext3 (data=ordered or writeback). - The launch line : /usr/bin/kvm -M pc-0.11 -m 512 -smp 1 -name kvm-ubuntu910 -uuid e5a362c5- c28a-93dd-043b-d46eb4daba37 -monitor unix:/var/run/libvirt/qemu/kvm- ubuntu910.monitor,server,nowait -boot c -drive file=/dev/storage-local-vol2- lvm/kvm-ubuntu910,if=virtio,index=0,cache=<cache_option>,boot=on -drive file=/dev/storage-local-vol2-lvm/bench,if=virtio,cache=<cache_option>,index=1 - k fr I have done tests with cache='none', cache='writeback' and cache='writethrough'. Everytime, the results was différents but in the same way : in the VM, IO Disk performance are better for little files and blocks and worst for big files and blocks. RHEL5.4 ============================== Similar tests ... Similar behaviour ... Host : RHEL5.4, ext3, KVM from Redhat; lvm dedicated partitions; driver disk from Redhat Guest : RHEL5.4, ext3, KVM from Redhat; lvm dedicated partitions; driver disk from Redhat -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
