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SubjectRe: [PATCH] 2.4.x write barriers (updated for ext3)
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sct@redhat.com said:
> Also, as soon as we have journals on external devices, this whole
> thing changes entirely. We still have to enforce the commit ordering
> in the journal, but we also still need the ordering between that
> commit and any subsequent writeback, and that obviousy can no longer
> be achieved via ordered tags if the two writes are happening on
> different devices.

Yes, that's a killer: ordered tags aren't going to be able to enforce cross
device write barriers.

There is one remaining curiosity I have, at least about the benchmarks: Since
the linux elevator and tag queueing perform essentially similar function
(except that the disk itself has a better notion of ordering because it knows
its own geometry). Might we get better performance by reducing the number of
tags we allow the device to use, thus forcing the writes to remain longer in
the linux elevator?

James


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