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

To: "Jens Axboe" <axboe@xxxxxxx>, "Kurt Garloff" <garloff@xxxxxxx>
Subject: RE: RE: RE: [Xen-devel] poor domU VBD performance.
From: "Ian Pratt" <m+Ian.Pratt@xxxxxxxxxxxx>
Date: Wed, 30 Mar 2005 12:16:10 +0100
Cc: Vincent Hanquez <tab@xxxxxxxxx>, Xen development list <xen-devel@xxxxxxxxxxxxxxxxxxx>, Christian Limpach <Christian.Limpach@xxxxxxxxxxxx>
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Thread-topic: RE: RE: [Xen-devel] poor domU VBD performance.
> I'll check the xen block driver to see if there's anything 
> else that sticks out.
> Jens Axboe

Jens, I'd really appreciate this.

The blkfront/blkback drivers have rather evolved over time, and I don't
think any of the core team fully understand the block-layer differences
between 2.4 and 2.6. 

There's also some junk left in there from when the backend was in Xen
itself back in the days of 1.2, though Vincent has prepared a patch to
clean this up and also make 'refreshing' of vbd's work (for size
changes), and also allow the blkfront driver to import whole disks
rather than paritions. We had this functionality on 2.4, but lost it in
the move to 2.6.

My bet is that it's the 2.6 backend that is where the true perofrmance
bug lies. Using a 2.6 domU blkfront talking to a 2.4 dom0 blkback seems
to give good performance under a wide variety of circumstances. Using a
2.6 dom0 is far more pernickety. I agree with Andrew that I suspect it's
the work queue changes are biting us when we don't have many outstanding


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