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Re: [Xen-devel] poor domU VBD performance.

To: Andrew Theurer <habanero@xxxxxxxxxx>
Subject: Re: [Xen-devel] poor domU VBD performance.
From: Kurt Garloff <garloff@xxxxxxx>
Date: Wed, 30 Mar 2005 01:19:47 +0200
Cc: Ian Pratt <m+Ian.Pratt@xxxxxxxxxxxx>, Christian Limpach <Christian.Limpach@xxxxxxxxxxxx>, Xen development list <xen-devel@xxxxxxxxxxxxxxxxxxx>, Jens Axboe <axboe@xxxxxxx>, Vincent Hanquez <tab@xxxxxxxxx>
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Hi Andrew,

On Tue, Mar 29, 2005 at 04:59:18PM -0600, Andrew Theurer wrote:
> On Tuesday 29 March 2005 16:45, Kurt Garloff wrote:
> > Please test attached patch.
> This should help the case where one is doing buffered IO (so readahead gets 
> used) but for o_direct, I still think we will have a problem.  On Dom0, I can 
> drive 58MB/sec with sequential read with o_direct with just a 32k request 
> size, but on domU with the same request size I can only get ~6MB/sec.

I can't reproduce this.
Does this depend on whether your domU root is a loopback mounted file
or a real partition/LVM device?

> I am still wondering is somthing is up with the backend driver.  It
> apperas that the backend driver only submits requests to the actual
> device every 10ms.  With a much larger request size (for o_direct) or
> a large readahead, 10ms is often enough to keep the disk streaming
> data.  With smaller request sizes or small read ahaad, the disk just
> doesn't read effciently.  

We might have a problem with unplugging then.

Kurt Garloff, Director SUSE Labs, Novell Inc.

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