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    On Wed, Mar 07 2001, Stephen C. Tweedie wrote:
    > > SCSI has ordered tag, which fit the model Alan described quite nicely.
    > > I've been meaning to implement this for some time, it would be handy
    > > for journalled fs to use such a barrier. Since ATA doesn't do queueing
    > > (at least not in current Linux), a synchronize cache is probably the
    > > only way to go there.
    > Note that you also have to preserve the position of the barrier in the
    > elevator queue, and you need to prevent LVM and soft raid from
    > violating the barrier if different commands end up being sent to
    > different disks.

    Yep, it's much harder than it seems. Especially because for the barrier
    to be really useful, having inter-request dependencies becomes a
    requirement. So you can say something like 'flush X and Y, but don't
    flush Y before X is done'.

    Jens Axboe

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