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

[Stefan Hajnoczi <stefanha@gmail.com>]

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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

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