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From: Paul Brook <paul@codesourcery.com>
Subject: Re: [Qemu-devel] [RFC] Disk integrity in QEMU
Date: Fri, 10 Oct 2008 13:53:31 +0100
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To: qemu-devel@nongnu.org
Cc: Chris Wright <chrisw@redhat.com>, Mark McLoughlin <markmc@redhat.com>, kvm-devel <kvm-devel@lists.sourceforge.net>, Laurent Vivier <Laurent.Vivier@bull.net>, Ryan Harper <ryanh@us.ibm.com>

> But that said, I'm concerned that this is far too confusing for users.
> I don't think anyone is relying on disk write performance when in
> write-back mode simply because the guest already has a page cache so
> writes are already being completed instantaneously from the
> application's perspective.

This isn't entirely true. With IDE devices you don't have command queueing, so 
it's easy for a large write to stall subsequent reads for a relatively long 
time.
I'm not sure how much this effects qemu, but I've definitely seen it happening 
on real hardware.

Paul