geom <-> cam disk

[Andriy Gapon]

on 26/07/2012 01:08 Alexander Motin said the following:
> Different controllers have different command queueing limitations. If you are
> testing with ahci(4) driver and modern disks, then their 32 command slots per
> port can be enough for many workloads to enqueue all commands to the hardware
> and leave queue empty as you've described. But if you take harder workload, or
> controller/ device without command queueing support, extra requests will be
> accumulated on that bioq and sorted there.

Alexander,

thank you for the reply.
Indeed, using 64 parallel dd processes with bs=512 I was able to 'kick in' the
disksort logic.  But I am not sure if the disksort algorithm makes much
difference in this case given the number of commands that a disk firmware can
internally re-order.  (Not mentioning that potentially disksort could starve
some I/O bound processes in favor of others -- but that's a totally different
topic).

But then, of course, for the less capable hardware the disksort could still be a
significant factor.

-- 
Andriy Gapon