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    SubjectRe: [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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