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



On Sat, Jun 02 2007, Tejun Heo wrote:
> 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?

Yeah, that's what the zero-length barrier implementation I posted does.
Not sure if you have a question beyond that, if so fire away :-)

-- 
Jens Axboe


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