Re: scsi vs ide performance on fsync's

[Jens Axboe <axboe@suse.de>]

On Wed, Mar 07 2001, Stephen C. Tweedie wrote:
> > Yep, it's much harder than it seems. Especially because for the barrier
> > to be really useful, having inter-request dependencies becomes a
> > requirement. So you can say something like 'flush X and Y, but don't
> > flush Y before X is done'.
> 
> Yes.  Fortunately, the simplest possible barrier is just a matter of
> marking a request as non-reorderable, and then making sure that you
> both flush the elevator queue before servicing that request, and defer
> any subsequent requests until the barrier request has been satisfied.
> One it has gone through, you can let through the deferred requests (in
> order, up to the point at which you encounter another barrier).

The above should have been inter-queue dependencies. For one queue
it's not a big issue, you basically described the whole sequence
above. Either sequence it as zero for a non-empty queue and make
sure the low level driver orders or flushes, or just hand it directly
to the device.

My bigger concern is when the journalled fs has a log on a different
queue.

> Only if the queue is empty can you give a barrier request directly to
> the driver.  The special optimisation you can do in this case with
> SCSI is to continue to allow new requests through even before the
> barrier has completed if the disk supports ordered queue tags.  

Yep, IDE will have to pay the price of a flush.

-- 
Jens Axboe