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Date: Tue, 14 Apr 2015 12:31:39 +0200
From: Kevin Wolf <kwolf@redhat.com>
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Subject: Re: [Qemu-devel] Very poor IO performance which looks like some
 design problem.
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To: ein <ein.net@gmail.com>
Cc: Fam Zheng <famz@redhat.com>, qemu-devel@nongnu.org

Am 13.04.2015 um 14:28 hat ein geschrieben:
> Dear Fam,
> 
> Check out my update please:
> http://lists.nongnu.org/archive/html/qemu-devel/2015-04/msg01318.html
> 
> Using aio=native,cache=none results in 500%-2000% performance drop comparing to
> bare metal and 300%-1000% comparing to aio=threads,cache=unsafe.

cache=unsafe isn't really useful to compare against because it throws
all flush requests away. If your benchmark includes disk flushes, you're
only cheating yourself with this mode.

It's generally accepted that cache=none,aio=native is the best
configuration for such scenarios. Also make sure to use the right guest
configuration; for example, using the right I/O scheduler can make a
major difference for Linux guests. Not sure how to configure Windows for
best performance, maybe someone else can help with that.

If you insist on cheating and would like to combine both, you could give
'cache=none,cache.no-flush=on,aio=native' a try.

Kevin