Re: [Qemu-devel] poor virtio-scsi performance

[Andrey Korolyov]

On Mon, Jun 8, 2015 at 12:52 PM, Alexey <address@hidden> wrote:
> Hi all!
>
> I suspected poor performance of virtio-scsi driver.
> I did a few tests:
>    Host machine: linux 3.19.1, QEMU emulator version 2.3.0
>    Guest machine: linux 4.0.4
>
>    part of domain xml:
>     <emulator>/usr/bin/kvm</emulator>
>     <disk type='block' device='disk'>
>       <driver name='qemu' type='raw' cache='none' io='native'
> discard='unmap'/>
>       <source dev='/dev/ram0'/>
>       <backingStore/>
>       <target dev='sda' bus='scsi'/>
>       <alias name='scsi0-0-0-1'/>
>       <address type='drive' controller='0' bus='0' target='0' unit='1'/>
>     </disk>
>
> /dev/ram0 I got by running `modprobe brd rd_size=$((5*1024*1024))` on host
> machine.
>
> fio conf:
>   [readtest]
>   blocksize=4k
>   filename=/dev/sdb (/dev/ram0 whe test from host machine)
>   rw=randread
>   direct=1
>   buffered=0
>   ioengine=libaio
>   iodepth=32
>
>
> results:
>   from host:
>     bw=1594.6MB/s, iops=408196, clat=76usec
>   from guest:
>     bw=398MB/s, iops=99720, clat=316usec
>
> Both host and guest system I boot with `scsi_mod.use_blk_mq=Y`.
>
> Why difference in 4 times?!
>
>
> Regards,
> Alexey Mochkin
>


You are probably hitting the (more or less fundamental) ceiling which
was the reason for introducing dataplane backend recently, in other
words, it does not matter how fast your backend is, the operation
number will be limited by 50..100kIOPS without dataplane. As far as I
remember the recent development made possible using it with
virtio-scsi as well as with virtio-blk, so you`d possibly want to try
it.