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

Pekka Enberg
2011-07-10T18:08:09Z

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.

Pekka