Hello,

Jens Axboe wrote:

quoted

Would that be very different from issuing barrier and not waiting for
its completion?  For ATA and SCSI, we'll have to flush write back cache
anyway, so I don't see how we can get performance advantage by
implementing separate WRITE_ORDERED.  I think zero-length barrier
(haven't looked at the code yet, still recovering from jet lag :-) can
serve as genuine barrier without the extra write tho.

As always, it depends :-)

If you are doing pure flush barriers, then there's no difference. Unless
you only guarantee ordering wrt previously submitted requests, in which
case you can eliminate the post flush.

If you are doing ordered tags, then just setting the ordered bit is
enough. That is different from the barrier in that we don't need a flush
of FUA bit set.

Hmmm... I'm feeling dense.  Zero-length barrier also requires only one
flush to separate requests before and after it (haven't looked at the
code yet, will soon).  Can you enlighten me?

Thanks.

-- 
tejun