Re: [Xen-devel] databases and xen?

[Ian Pratt <Ian.Pratt@xxxxxxxxxxxx>]

> 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).


Ian


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