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Re: [Xen-devel] bad write performance with qdisk with larger files in pv-domU

On Fri, Jun 03, 2011 at 11:49:47PM +0000, Ronny Hegewald wrote:
> Im using the following 32-bit setup:
> - xen 4.1.0 
> - upstream linux-kernel 2.6.39 as dom0
> - linux 2.6.32 pv-domU that has several ext3 partitions mounted with qdisk 
> (same behaviour with a 2.6.39 kernel, so i continued the investigation with 
> the 2.6.32er kernel)
> Die read performance is good (ca. 60 MB/s)
> For smaller files (< 30-40 MB) the write-speed is ok.
> But if i copy a larger file (ca > 40 MB), the write speed decreases to ca. 
> 0,5 
> MB/s, after the first ca. 40 MBs are written.
> One reason for the bad performance might be that qdisk doesnt use AIO. For 
> testing purposes i activated AIO in hw/xen_disk.c (i set use_aio=1), but the 
> domU freezed shortly after the domU-kernel started.

You could also use this patch:

But why not use the 3.0-rc1 with the xen-blkback? Or if you want to use 2.6.39
you could use the 

git://git.kernel.org/pub/scm/linux/kernel/git/konrad/xen.git stable/2.6.39.x

> Is this performance-impact expected when no AIO is used? 

Yeah, it is slow.
> I compared the raw-block implementation from xen-qemu 4.1.0 and current 
> upstream, in case xen-qemu has some missing bugfixes and found the following 
> patch that looks a bit interesting
>       commit 4899d10d142e97eea8f64141a3507b2ee1a64f52
>       Author: Stefan Hajnoczi <stefanha@xxxxxxxxxxxxxxxxxx>
>       Date:   Mon Apr 19 13:34:11 2010 +0100
>       raw-posix: Use pread/pwrite instead of lseek+read/write
>       This patch combines the lseek+read/write calls to use pread/pwrite
>       instead.  This will result in fewer system calls and is already used by
>       AIO.
> From the first look the patch cannot be backported 1:1, so i havent tried it 
> yet, because i doubt that it can make such a huge difference. Or would it be 
> worth a try?
> Any other ideas how/what to investigate this issue further, in case the write-
> speed should be better also without AIO? I know that the qdisk implementation 
> is expected to be slower, but i would expect at least lets say 5 MB/s.
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