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

[Ming Lei]

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.


Thanks,
--
Ming Lei