[dm-devel] Re: [RFD] BIO_RW_BARRIER - what it means for devices, filesystems, and dm/md.

[Tejun Heo]

Hello,

Jens Axboe wrote:
>> 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