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Re: Darwin disk I/O question

On Thursday, Jan 2001 at 8:55:3 Dave Yost wrote:
> I am seeking a disk driver expert who knows about the following:
>
> I know classically unix has taken the position that disk drivers are expecte
> d to keep the request queue sorted by cylinder and sector position to minimi
> ze total time spent waiting for seek and rotational latency, i.e. to maximiz
> e 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 wou
> ld there have to be a change to the driver protocol?

After taking a recent EE course from a guy from Quantum, there is no
such thing as CHS anymore from the disk's perspective. The disk really
just makes those up and hands them to the OS/BIOS/Firmware as the normal
interface to the disk. The disk really just takes these and maps them
to it's internal structure that is almost always totally different.
These days, the old CHS system is merely an interface to the disk that
bears no resemblance to reality. One should not try to optimize
disk requests on the assumption that these things match the physical
layout of the disk. It is far better to just barrage the disk with
requests and let it sort them out.

I believe the current disk queue is not sorted with respect to data
location, or with respect to the process' priority.

Rob



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