Mark Hahn wrote:
I though that NCQ was intended to increase performance ??
intended to increase _sales_ performance ;)
Yep.
remember that you've always had command queueing (kernel elevator): the
main difference with NCQ (or SCSI tagged queueing) is when
the disk can out-schedule the kernel. afaikt, this means sqeezing
in a rotationally intermediate request along the way.
that intermediate request must be fairly small and should be a read
(for head-settling reasons).
I wonder how often this happens in the real world, given the relatively
small queues the disk has to work with.
ISTR either Jens or Andrew ran some numbers, and found that there was
little utility beyond 4 or 8 tags or so.
My hdparm test is a sequential read-ahead test, so it will
naturally perform worse on a Raptor when NCQ is on.
that's a surprisingly naive heuristic, especially since NCQ is concerned
with just a max of ~4MB of reads, only a smallish
fraction of the available cache.
NCQ mainly helps with multiple threads doing reads. Writes are largely
asynchronous to the user already (except for fsync-style writes). You
want to be able to stuff the disk's internal elevator with as many read
requests as possible, because reads are very often synchronous -- most
apps (1) read a block, (2) do something, (3) goto step #1. The kernel's
elevator isn't much use in these cases.
Jeff
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