Re: [PATCH 0/9] kvm tools, qcow: Improve QCOW performance

Ingo Molnar
2011-07-10T18:17:59Z

* Pekka Enberg <penberg@kernel.org> wrote:

> Hi Ingo,
>
> * Pekka Enberg <penberg@kernel.org> 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@elte.hu> 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?

Thanks,

Ingo