Re: Xen Dom0 vs. DomU disk I/O

[Manuel Bouyer <bouyer%antioche.eu.org@localhost>]

On Fri, Nov 23, 2012 at 02:46:18PM +0100, dieter roelants wrote:
> With a 6.0_STABLE from early november on a sata disk dd'ing 1G:
> 
> On the dom-0:
> to a file on ffs: 93M/s.
> to a raw partition: 96M/s.
> 
> On the dom-u:
> to a file on ffs: 28M/s.
> to raw partition: 97M/s.
> 
> When writing to the dom-0's file system I get 64k / transfer, for
> the dom-u it's only 32k.

this can be the problem. The PV disk interface allows only 32k transfers
between the domU and dom0. There is code to coalesce writes, but the
domU has to post enough writes at once for the dom0 to be able
to coalece.

this needs to be looked at.

-- 
Manuel Bouyer <bouyer%antioche.eu.org@localhost>
     NetBSD: 26 ans d'experience feront toujours la difference
--