On Wed, Jul 2, 2014 at 4:56 PM, Paolo Bonzini <pbonz...@redhat.com> wrote: > Il 02/07/2014 10:39, Ming Lei ha scritto: > >> Then start to read payload in original path, but no plug/unplug any >> more. Also another request may follows, and another plug&unplug >> comes too, which makes thing more complicated, so I suggest to >> enable plug&unplug only for raw driver now. > > > That's just a performance issue (and actually one that wasn't in 2.0 because > qcow2 on dataplane wasn't supported there). In many cases the cache hit of > the qcow2 metadata cache can be very high, and avoiding plug/unplug would > prevent an easy performance bonus.
I just gave a test on qcow2 image with dataplane on, and looks it can work, so I will take original way to implement the api. But making full use of plug/unplug for non-raw formats need changes in format drivers in future. > > I don't especially like plug/unplug as an API (I think it's better to extend > aio_multiwrite to include other kind of requests), but: > > - either we have qualms on the correctness of it, and then we should live > with the regressions > > - or if the patches are not messy and reverting them is easy, we should go > for it. This is what we did for dataplane in the first place, and we can > keep doing it in the 2.1 dataplane code. OK, let's keep the API's as it now. Actually I have thoughts in mind for plug&unplug changes too, and now it is better to make it a bit simple, > >> > If you could make it an assertion, that would also be great. (BTW, it's >> > probably best to add a nesting count to plug/unplug). >> >> I'd rather to add it in future if we really need that. > > > I think this is a matter of correctness as Kevin's review pointed out. OK, specifically it is needed for non-raw format driver. Thanks, -- Ming Lei
