[Qemu-discuss] virtio-console downgrade virtio-pci-blk performance

[Long Zhang]

Hi all,

We tried to install qemu-ga on our guest vm and found an significant
performance downgrade on disk I/O when performing fio tests after
virtio-console module installation. The downgrade went away after removing
virtio-console module.

This issue exists in nearly all Qemu versions and all Linux

(CentOS7, Fedora 28, Ubuntu 18.04) distros as we tested.

This is a disk cmd:

-drive file=iscsi://
127.0.0.1:3260/iqn.2016-02.com.test:system:fl-iscsi/1,format=raw,if=none,id=drive-virtio-disk0,cache=none,aio=native

-device
virtio-blk-pci,scsi=off,bus=pci.0,addr=0x6,drive=drive-virtio-disk0,id=virtio-disk0,bootindex=1,write-cache=on

This is the FIO test:

[global]

ioengine=libaio

iodepth=128

runtime=120

time_based

direct=1

[randwrite-a-256k-seq]

stonewall

bs=4k

filename=/dev/vdb

rw=randwrite

Device /dev/vdb is a empty drive mounted for disk I/O throughput test.

If I add "-device

virtio-serial-pci,id=virtio-serial0,bus=pci.0,addr=0x5 ", the virtio

disk 4k iops (randread/randwrite) would downgrade from 60k to 40k.

Inside guest VM, if I rmmod virtio-console, the performance will back to
normal.

Any idea about this issue? I don't know this is a qemu issue or kernel
issue.

Thanks in advance!