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

From: George Dunlap <george.dunlap@eu.citrix.com>
To: Sylvain Munaut <s.munaut@whatever-company.com>
Cc: James Harper <james.harper@bendigoit.com.au>,
	Wei Liu <Wei.Liu2@citrix.com>,
	Stefano Stabellini <stefano.stabellini@eu.citrix.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 15:04:47 +0100	[thread overview]
Message-ID: <51FFB0FF.7070607__15777.5537962609$1375711646$gmane$org@eu.citrix.com> (raw)
In-Reply-To: <CAF6-1L6BJh3OMsCXocuTd_VV0orD2sUgP7KqD1hZhRyMeRO17A@mail.gmail.com>

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?

>
>
>> One of the reasons for stopping work on blktap3 (AIUI) was that it
>> should in theory have performance characteristics similar to blktap3,
> And did anyone check the theory currently ? :)

I say "in theory" because they are using the same basic architecture: a 
normal process running in dom0, with no special kernel support.  If 
there were a performance difference, it would be something that should 
(in theory) be able to be optimized.

I don't think we have comparisons between qdisk (which is what we call 
qemu-as-pv-backend in Xen) and blktap3 (and since blktap3 wasn't 
finished they wouldn't mean much anyway); but I think qdisk compares 
reasonably with blkback.

  -George