Re: [Qemu-devel] [PATCH] qcow2: improve I/O performance with cache=off

[Laurent Vivier <Laurent.Vivier@bull.net>]

Le mardi 24 juin 2008 à 17:40 +0200, Kevin Wolf a écrit :
> Hi Laurent,

Hi Kevin,

> Laurent Vivier schrieb:
> > this patch improves qcow2 I/O performance when used with cache=off.
> 
> Why do you think this patch helps only for cache=off? I have applied
> your patch to Xen ioemu (which has no cache=off / O_DIRECT yet) and
> I certainly do see a performance gain for large block sizes (using dd).
> With small block sizes like 512 bytes or 1k I lose a bit of perfomance,
> though.

In fact I made some tests with dbench and results were not as good as
with cache=off. It's why I spoke only about cache=off. But as said Avi,
dbench is not a good benchmark for this...

                                   WITHOUT          WITH

ide, cache=off,snapshot=off     20.8494 MB/sec  24.0711 MB/sec
ide, cache=off,snapshot=on      20.9349 MB/sec  24.5031 MB/sec
ide, cache=on, snapshot=off     23.6612 MB/sec  24.7186 MB/sec
ide, cache=on, snapshot=on      24.1836 MB/sec  24.7678 MB/sec
scsi,cache=off,snapshot=off     21.0264 MB/sec  24.6119 MB/sec
scsi,cache=off,snapshot=on      21.4184 MB/sec  24.6739 MB/sec
scsi,cache=on, snapshot=off     25.1483 MB/sec  24.8600 MB/sec
scsi,cache=on, snapshot=on      25.2000 MB/sec  25.2758 MB/sec

> bonnie++ shows slightly better numbers with this patch, too. In the
> case of block reads the improvement is huge and I even got double
> throughput.
> I also had a look at your code and it seems fine to me. (Except that
> the aio callback handlers become even longer, but that is a different
> problem...)

I modify this patch according Avi comments, and I'll repost it.

Thank you for your comments.

Regards,
Laurent
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