Am 10.07.2011 20:08, schrieb Pekka Enberg: > Hi Ingo, > > * Pekka Enberg <penberg@xxxxxxxxxx> wrote: >>> This series fixes QCOW locking issues and implements delayed metadata writeout. >>> This improves performance of writeout to QCOW2 images that don't have clusters >>> and L2 tables allocated on-disk. >>> >>> I tested the series by running >>> >>> mount -t ext4 /dev/vdb /mnt >>> dd if=/dev/zero of=/mnt/tmp >>> >>> in the guest multiple times for fresly generated QCOW2 image: >>> >>> dd if=/dev/zero of=fs.ext4 bs=1024k count=512 && mkfs.ext4 -F fs.ext4 && qemu-img convert -O qcow2 fs.ext4 fs.qcow2 >>> >>> which causes worst-case behavior for the current code. >>> >>> Before: >>> >>> [ seekwatcher: http://userweb.kernel.org/~penberg/kvm-qcow-delayed/kvm-qcow2-master.png ] >>> >>> 511229952 bytes (511 MB) copied, 19.906 s, 25.7 MB/s >>> 511229952 bytes (511 MB) copied, 20.3168 s, 25.2 MB/s >>> 511229952 bytes (511 MB) copied, 20.8078 s, 24.6 MB/s >>> 511229952 bytes (511 MB) copied, 21.0889 s, 24.2 MB/s >>> 511229952 bytes (511 MB) copied, 20.7833 s, 24.6 MB/s >>> 511229952 bytes (511 MB) copied, 20.7536 s, 24.6 MB/s >>> 511229952 bytes (511 MB) copied, 20.0312 s, 25.5 MB/s >>> >>> After: >>> >>> [ seekwatcher: http://userweb.kernel.org/~penberg/kvm-qcow-delayed/kvm-qcow2-delayed.png ] >>> >>> 511229952 bytes (511 MB) copied, 7.68312 s, 66.5 MB/s >>> 511229952 bytes (511 MB) copied, 7.54065 s, 67.8 MB/s >>> 511229952 bytes (511 MB) copied, 9.34749 s, 54.7 MB/s >>> 511229952 bytes (511 MB) copied, 9.2421 s, 55.3 MB/s >>> 511229952 bytes (511 MB) copied, 9.9364 s, 51.5 MB/s >>> 511229952 bytes (511 MB) copied, 10.0337 s, 51.0 MB/s >>> 511229952 bytes (511 MB) copied, 9.39502 s, 54.4 MB/s > > On Sun, Jul 10, 2011 at 8:15 PM, Ingo Molnar <mingo@xxxxxxx> wrote: >> Just wondering, how does Qemu perform on the same system using the >> same image, with comparable settings? > > Freshly built from qemu-kvm.git: > > $ /home/penberg/qemu-kvm/x86_64-softmmu/qemu-system-x86_64 --version > QEMU emulator version 0.14.50 (qemu-kvm-devel), Copyright (c) > 2003-2008 Fabrice Bellard > > Tests were run with this configuration: > > $ /home/penberg/qemu-kvm/x86_64-softmmu/qemu-system-x86_64 -kernel > /boot/vmlinuz-3.0.0-rc5+ -drive > file=/home/penberg/images/debian_squeeze_amd64_standard.img,if=virtio,boot=on > -drive file=fs.qcow2,if=virtio -nographic -m 320 -smp 2 -append > "root=/dev/vda1 console=ttyS0 init=/root/iobench-write" > > Not sure if that's 100% comparable settings but anyway. The results > looks as follows: I would love to try out your code occasionally myself, but so far I have been to lazy to build a guest kernel only to be able to test it. Having to deal with the huge kernel git tree just for a small program doesn't really make it more fun either... Anyway, what I'm trying to say is that everything in my mails is from a purely theoretical POV. I have only looked at the code, but never really tried it. As Ingo already said, the cache mode is probably the major difference. >From what I can see in your code, cache=writeback would be the equivalent for what tools/kvm is doing, however cache=none (i.e. O_DIRECT) is what people usually do with qemu. And then there seems to be another big difference. I hope I'm not missing anything, but you seem to be completely lacking refcount handling for qcow2. This is okay for read-only image, but with write access to the image, you're corrupting the images if you don't update the refcounts. Have you checked qcow2 images with qemu-img check after tools/kvm having written to it? Maintaining the right order between L2 writes and refcount block writes is another source of flushes in qemu, which of course makes a difference for performance. Kevin -- 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
