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    SubjectRe: [RFD] BIO_RW_BARRIER - what it means for devices, filesystems, and dm/md.
    Jens Axboe wrote:
    > 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 :-)

    I thought you were talking about adding BIO_RW_ORDERED instead of
    exposing zero length BIO_RW_BARRIER. Sorry about the confusion. :-)

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