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

[Paul Brook <paul@codesourcery.com>]

> 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.

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.

Paul