In article <20000501151939.H11600@redhat.com> you wrote:
> Hi,
>> Elevator sorting offers little or no benefit to higher performance I./O
>> devices - An
>> extreme example would be RAID controllers that have on card memory that
> True, but only if the memory survives a reboot. However, even in
> these situations, the elevator serves a useful purpose --- it allows
> us to merge adjacent IO requests together in the queue, letting us
> reduce the number of hardware IOs we generate.
My experiments have shown that a typical (ie the current) elevator merges
about 70% of the requests, while a "simple" elevator that only tries to
merge the previous request (without the sorting) merges about 60%. So the
gain of sorting for merging is just 10%points.
Another issue with RAID: If two adjacent requests (that could be merged)
fall in different 64Kb blocks, a RAID array might be able to processes the
second request because the relevant disk for that request is idle, while
after the merge the request wouldn't be processed "out of order". It would
even have to be split again.
Greetings,
Arjan van de Ven
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