Re: [Qemu-devel] I/O performance degradation with Virtio-Blk-Data-Plane

[Stefan Hajnoczi]

On Mon, Sep 02, 2013 at 05:24:09PM +0900, Jonghwan Choi wrote:
> Nowdays i measured io performance with Virtio-Blk-Data-Plane.
> There was something strange in test.
> When vcpu count is 1, io performance is increased in test
> But vcpu count is over 2, io performance is decreased in test.
> 
> i used 3.10.9 stable kernel, qemu(1.4.2) & fio 2.1
> 
> What should i check in my test?

It's hard to say without any details on your benchmark configuration.

In general, x-data-plane=on performs well with SMP guests and with
multiple disks.  This is because the dataplane threads can process I/O
requests without contending on the QEMU global mutex or the iothread
event loop.

In order to investigate further, please post:

1. The fio results so it's clear which cases performed worse and by how
   much.

2. The fio job files.

3. The QEMU command-line to launch the guest.

4. The host disk configuration (e.g. file systems, local
   SATA/FibreChannel/NFS, etc).

5. The basic host specs including RAM and number of logical CPUs.

Stefan