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

[Andrew Theurer <habanero@xxxxxxxxxx>]

On Tuesday 29 March 2005 17:19, Kurt Garloff wrote:
> 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 not sure.  What program are you using for o_direct reads?  I use a real 
LVM device for domU root and then another whole disk for the read tests.

> > 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.

That's what I suspect, but I do not know the driver code well enough to say 
for sure.

-Andrew

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