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

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.


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