Hi all,

We found an significant performance downgrade on disk I/O performance after virtio-console module installation.

This situation exists in nearly all Qemu versions and all Linux
(CentOS7, Fedora 28, Ubuntu 18.04) distros.

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 case:
[global]
ioengine=libaio
iodepth=128
runtime=120
time_based
direct=1

[randwrite-a-256k-seq]
stonewall
bs=4k
filename=/dev/vdb
rw=randwrite

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. Feel free to let me know if further evidence is needed to isolate root cause.

Thanks in advance!

Thanks and Best Regards,
Alex