On Sun, Jul 10, 2011 at 9:17 PM, Ingo Molnar <mingo@xxxxxxx> wrote: > > * Pekka Enberg <penberg@xxxxxxxxxx> wrote: > >> 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: >> >> 511229952 bytes (511 MB) copied, 12.5543 s, 40.7 MB/s >> 511229952 bytes (511 MB) copied, 9.50382 s, 53.8 MB/s >> 511229952 bytes (511 MB) copied, 12.1092 s, 42.2 MB/s >> 511229952 bytes (511 MB) copied, 13.2981 s, 38.4 MB/s >> 511229952 bytes (511 MB) copied, 11.3314 s, 45.1 MB/s >> 511229952 bytes (511 MB) copied, 12.7505 s, 40.1 MB/s >> 511229952 bytes (511 MB) copied, 11.2921 s, 45.3 MB/s >> >> This is what I'd expect as tools/kvm has much more relaxed sync() >> guarantees than qemu-kvm. We treat all writes to QCOW2 images as >> volatile until VIRTIO_BLK_T_FLUSH is issued. Furthemore, for this >> particular (special case) load, it's pretty much append-only to the >> backing file which is why QCOW is so close to raw image performance >> here. > > Pretty impressive numbers! > > To relax Qemu's caching guarantees you can append ,cache=writeback to > your -drive option, i.e. something like: > > -drive file=/dev/shm/test.qcow2,cache=writeback,if=virtio > > Does that improve the Qemu results? Yes, it seems so: 511229952 bytes (511 MB) copied, 10.0879 s, 50.7 MB/s 511229952 bytes (511 MB) copied, 4.92686 s, 104 MB/s 511229952 bytes (511 MB) copied, 13.1955 s, 38.7 MB/s 511229952 bytes (511 MB) copied, 10.7322 s, 47.6 MB/s 511229952 bytes (511 MB) copied, 9.46115 s, 54.0 MB/s 511229952 bytes (511 MB) copied, 14.9963 s, 34.1 MB/s 511229952 bytes (511 MB) copied, 11.1701 s, 45.8 MB/s The numbers seem much more unstable from run to run with 'writeback' so it's pretty difficult to say how much it helps. I'm doing 'drop_caches' after image creation so I don't quite understand why they are so unstable. Pekka -- 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
