Re: [Qemu-devel] [PATCH v2 1/3] block: block: introduce bdrv_io_plug() and bdrv_io_unplug()

[Kevin Wolf]

Am 02.07.2014 um 11:29 hat Ming Lei geschrieben:
> On Wed, Jul 2, 2014 at 5:12 PM, Kevin Wolf <address@hidden> wrote:
> > Am 02.07.2014 um 10:56 hat Paolo Bonzini geschrieben:
> >> 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 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.
> >
> > Fully agree. This series is small enough and obviously fixes a
> > dataplane problem, so at least for 2.1 we should go for it.
> >
> > My thoughts in the other mail were more about where to go in the long
> > term. We need to have a decision about what API we commit to - something
> > multiwrite-like or something plug/unplug-like - before we want to start
> > converting everything to that interface.
> >
> > This is why I think we should be thinking about how to implement certain
> > optimisations (like the request merging with plug/unplug, as I mentioned;
> > or mixing read and writes in one batch with multiwrite) in both models.
> > Only when we have a reasonbly good idea of what the result would look
> > like in either case we can make an informed decision.
> 
> Actually linux-aio can support to submit read/write to multi files, and
> virtio-scsi does have the use case, so in future io queue should be
> per aio-context as I posted 1st time.  And I am wondering if multiwrite-like
> APIs can fit in this situation.

Though where would you get the requests for two different files from,
within the same bdrv_plug/unplug block?

Hm, okay, I guess backing files might be a valid point. Anything else?

Kevin