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Re: [Xen-devel] Performance on QEMU IDE disks

To: Steve Ofsthun <sofsthun@xxxxxxxxxxxxxxx>
Subject: Re: [Xen-devel] Performance on QEMU IDE disks
From: Stephen Tweedie <sct@xxxxxxxxxx>
Date: Tue, 18 Jul 2006 10:49:30 -0400
Cc: xen-devel@xxxxxxxxxxxxxxxxxxx, "P M, Priya \(STSD\)" <pm.priya@xxxxxx>
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On Fri, Jul 14, 2006 at 03:41:23PM -0400, Steve Ofsthun wrote:

> To fix this you would need to patch QEMU to used O_DIRECT when accessing
> the virtual disk backing store object (block device, file, etc).  We are
> currently testing just such a patch for the old QEMU device model.


>  I
> haven't looked at the new device model to see if it already handles this.

I looked last week and it still had the same problem.

Note that O_DIRECT doesn't solve all of the problems either.  You
still end up with the _entire_ device model being blocked while the
disk IO is in progress, which will leads to poor VT guest performance
when trying to mix disk and CPU activity.  But I'd rather have a
performance problem than a data corrupter. :)


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