Re: [Xen-devel] Xen blktap driver for Ceph RBD : Anybody wants to test ? :p

[George Dunlap <george.dunlap@eu.citrix.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