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From: Alexandre DERUMIER <aderumier@odiso.com>
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Subject: Re: [Qemu-devel] poor virtio-scsi performance
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To: Vasiliy Tolstov <v.tolstov@selfip.ru>
Cc: libvir-list@redhat.com, qemu-devel <qemu-devel@nongnu.org>

Hi,
if you want to use multiqueues in guest, you need to enabled it on virtio-s=
csi controller.

<controller type=3D'scsi' index=3D'0' model=3D'virtio-scsi' num_queues=3D'8=
'/>

for example.


----- Mail original -----
De: "Vasiliy Tolstov" <v.tolstov@selfip.ru>
=C3=80: "qemu-devel" <qemu-devel@nongnu.org>, libvir-list@redhat.com
Envoy=C3=A9: Lundi 8 Juin 2015 12:30:59
Objet: [Qemu-devel] poor virtio-scsi performance

Hi all!=20

I suspected poor performance of virtio-scsi driver.=20
I did a few tests:=20
Host machine: linux 3.19.1, QEMU emulator version 2.3.0=20
Guest machine: linux 4.0.4=20

part of domain xml:=20
<emulator>/usr/bin/kvm</emulator>=20
<disk type=3D'block' device=3D'disk'>=20
<driver name=3D'qemu' type=3D'raw' cache=3D'none' io=3D'native' discard=3D'=
unmap'/>=20
<source dev=3D'/dev/ram0'/>=20
<backingStore/>=20
<target dev=3D'sda' bus=3D'scsi'/>=20
<alias name=3D'scsi0-0-0-1'/>=20
<address type=3D'drive' controller=3D'0' bus=3D'0' target=3D'0' unit=3D'1'/=
>=20
</disk>=20

/dev/ram0 I got by running `modprobe brd rd_size=3D$((5*1024*1024))` on=20
host machine.=20

fio conf:=20
[readtest]=20
blocksize=3D4k=20
filename=3D/dev/sdb (/dev/ram0 whe test from host machine)=20
rw=3Drandread=20
direct=3D1=20
buffered=3D0=20
ioengine=3Dlibaio=20
iodepth=3D32=20


results:=20
from host:=20
bw=3D1594.6MB/s, iops=3D408196, clat=3D76usec=20
from guest:=20
bw=3D398MB/s, iops=3D99720, clat=3D316usec=20

Both host and guest system I boot with `scsi_mod.use_blk_mq=3DY`.=20

Why difference in 4 times?!=20

--=20
Vasiliy Tolstov,=20
e-mail: v.tolstov@selfip.ru=20