[Qemu-devel] poor virtio-scsi performance

[Alexey <alukardd@alukardd.org>]

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