Re: [Qemu-devel] [RFC][PATCH] make sure disk writes actually hit disk

[Jens Axboe]

On Sat, Jul 29 2006, Paul Brook wrote:
> > Easy to do with the fsync infrastructure, but probably not worth
> > doing since people are working on the AIO I/O backend, which would
> > allow multiple outstanding writes from a guest.  That, in turn,
> > means I/O completion in the guest can be done when the data really
> > hits disk, but without a performance impact.
> 
> Not entirely true. That only works if you allow multiple guest IO
> requests in parallel, ie. some form of tagged command queueing. This
> requires either improving the SCSI emulation, or implementing SATA
> emulation. AFAIK parallel IDE doesn't support command queueing.

Parallel IDE does support queuing, but it never gained wide spread
support and the standard is quite broken as well (which is probably
_why_ it never got much adoption). It was also quite suboptimal from a
CPU efficiency POV.

Besides, async completion in itself is not enough, QEMU still needs to
honor ordered writes (barriers) and cache flushes.

> My impression what that the initial AIO implementation is just
> straight serial async operation. IO wouldn't actually go any faster,
> it just means the guest can do something else while it's waiting.

Depends on the app, if the io workload is parallel then you should see a
nice speedup as well (as QEMU is then no longer the serializing bottle
neck).

-- 
Jens Axboe