[libvirt] [Qemu-devel] poor virtio-scsi performance

[Alexandre DERUMIER]

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

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

for example.


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

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 at selfip.ru