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Re: Darwin disk I/O question

on 1/18/01 6:23 PM, email@hidden at
email@hidden wrote:

> Date: Thu, 18 Jan 2001 08:55:03 -0800
> To: email@hidden
> From: Dave Yost <email@hidden>
> Subject: Darwin disk I/O question
>
> I am seeking a disk driver expert who knows about the following:
>
> I know classically unix has taken the position that disk drivers are expected
> to keep the request queue sorted by cylinder and sector position to minimize
> total time spent waiting for seek and rotational latency, i.e. to maximize
> total disk throughput.
>
> Is this still true in Darwin? Or do Darwin disk drivers sort requests based
> on the requesting process's priority? Could they if they wanted to, or would
> there have to be a change to the driver protocol?
>
> Thanks
>
> Dave

Darwin's disk drivers are single-threaded (per device) and do not implement
any reordering of requests. This is left as an exercise for the developer.

Nowadays, I'd expect the disk drive firmware to do any optimization based on
current disc position. Also I'd expect the disk driver to be blissfully
ignorant of the requestor and its priority.

Stuart



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