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From: Vasiliy Tolstov <v.tolstov@selfip.ru>
Date: Mon, 8 Jun 2015 13:30:59 +0300
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Subject: [Qemu-devel] poor virtio-scsi performance
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To: qemu-devel <qemu-devel@nongnu.org>, libvir-list@redhat.com

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?!

-- 
Vasiliy Tolstov,
e-mail: v.tolstov@selfip.ru