Messages in this thread | ![/](/images/icornerl.gif) | | Date | Wed, 7 Mar 2001 15:12:41 +0100 | From | Jens Axboe <> |
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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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