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

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