Re: Xen blktap driver for Ceph RBD : Anybody wants to test ? :p

From: Wei Liu <wei.liu2@citrix.com>
To: George Dunlap <george.dunlap@eu.citrix.com>
Cc: James Harper <james.harper@bendigoit.com.au>,
	Wei Liu <wei.liu2@citrix.com>,
	Stefano Stabellini <stefano.stabellini@eu.citrix.com>,
	Sylvain Munaut <s.munaut@whatever-company.com>,
	"xen-devel@lists.xen.org" <xen-devel@lists.xen.org>,
	"ceph-devel@vger.kernel.org" <ceph-devel@vger.kernel.org>,
	Roger Pau Monne <roger.pau@citrix.com>
Subject: Re: Xen blktap driver for Ceph RBD : Anybody wants to test ? :p
Date: Mon, 5 Aug 2013 16:32:42 +0100	[thread overview]
Message-ID: <20130805153242.GH27858__3293.5235453353$1375716930$gmane$org@zion.uk.xensource.com> (raw)
In-Reply-To: <51FFC2B4.6010100@eu.citrix.com>

On Mon, Aug 05, 2013 at 04:20:20PM +0100, George Dunlap wrote:
> On 05/08/13 16:18, Wei Liu wrote:
> >On Mon, Aug 05, 2013 at 03:04:47PM +0100, George Dunlap wrote:
> >>On 05/08/13 14:55, Sylvain Munaut wrote:
> >>>Hi George,
> >>>
> >>>
> >>>>Yes; qemu knows how to be a Xen PV block back-end.
> >>>Very interesting. Is there documentation about this somewhere ?
> >>>I had a look some time ago and it was really not very clear.
> >>>
> >>>Things like what Xen version support this. And with which features (
> >>>indirect descriptors, persistent grants, discard, flush, ...) and/or
> >>>which limitation.
> >>I don't think this is documented anywhere; you'll need to ask the
> >>experts.  Stefano? Roger? Wei?
> >>
> >These are Linux features not Xen ones AFAICT. In theory they are not
> >bound to specific Xen versions.
> >
> >For the network part I don't think new features depend on any specific
> >hypercall. However for block Roger and Stefano seem to introduce
> >new hypercalls for certain features (I might be wrong though).
> 
> We're talking about qemu; so the toolstack needs to know how to set
> up qdisk, and I think qdisk would need to be programmed to use, for
> example, persistent grants, yes?
> 

I don't think toolstack needs to involve in this. At least for the
network part FE and BE negotiate what features to use. The general idea
is that new feature will always be of benifit to enable so we make use
of them whenever possible. Certain features do have sysfs entries to
configure but that's not coded into libxl.

I cannot speak for block drivers, but grepping the source code I don't
think you can configure persistent grants via libxl either.

Wei.

>  -G