Re: [Xen-devel] databases and xen?

[Matt Ayres <matta@xxxxxxxxxxxx>]

On Tue, 2005-01-18 at 20:45 +0000, Ian Pratt wrote:
> > On Mon, 17 Jan 2005, Wim Coekaerts wrote:
> > 
> > > testign it has already been fine. I ran 4 databases each in one domain
> > > (oracle10g) and it s been amazingly stable. I have not however done
> > > performance testing. soon...
> > 
> > How does O_DIRECT IO to xen block devices work ?
> 
> I'm not entirely sure quite what O_DIRECT is defined to do. It
> probably bypasses the guest's buffer cache OK, and gets turned
> into IO's in domain 0 without passing through the buffer cache
> (unless you're using a loop file or suchlike).
> 
> I believe IOs wont be acknowledged to the guest until they've
> been completed in dom0. However, they can be reordered. We
> probably need to add in a re-order barrier operation on the VBD
> device channel protocol in order to guarantee ordering where it
> matters (this would be marginally more efficient than
> holding back following requests in the guest).
> 

To avoid the buffercache problems of loopback images in dom0 you can
have dom0 open the loop files with O_DIRECT.  This should help
performance when using loop files as it eliminates the double buffering.

> 
> Ian
> 
> 
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