Re: [Qemu-devel] Greate difference of disk I/O performance for guest on Qemu-2.30 of CentOS.

[Stefan Hajnoczi]

On Mon, Jun 15, 2015 at 09:14:52PM +0800, cauchy-love wrote:
>    I am running a embedded OS on Qemu-2.3.0. However I found that the I/O 
> performance was quite different for different CentOS release (CentOS 6.5 and 
> CentOS 7.0). The CentOS 6.5 uses linux-2.6.32 while the CentOS 7.0 uses 
> linux-3.10.0. The I/O throughput (write) of the former (CentOS 6.5) is about 
> ten times of that on the latter (CentOS 7.0). All configurations are the same 
> except the kernel version. And the command line is as following:
>                       # qemu-kvm -m 2G -smp 4 -enable-kvm -hda guest.img ...
>  The I/O throughput of the host is almost the same for these two different 
> kernels, as tested by Iozone tool. Actually, I found that the IO throughput 
> degraded significantly since the 3.4 or later kernel version. I also tried 
> different qemu versions such as qemu-1.5.3, qemu-2.1.3 but got no 
> improvements.

Please post the iozone command-line you are using and the output.

Your QEMU command-line uses the IDE storage controller, which is not
optimized for performance.  Usually virtio-blk is used when good
performance is required:

  -drive if=virtio,cache=none,format=raw,aio=native,file=guest.img

The guest OS needs virtio-blk device drivers in order for this to work.

Stefan

pgpBoqLfqNzXD.pgp
Description: PGP signature