Risc System/6000

[jim frost]

mjacob at wonky.Sun.COM (Matt Jacob) writes:
>My own personal opinion is that geometry based filesystems are
>getting to be a bad microoptimization. With the coming of SCSI-2
>multiple command targets, it seems to me that one should just
>concentrate on getting requests out to the target as quickly
>as possible and let the microprocessor on the drive figure out
>the best order do them in.

You'd be wrong no matter what you did.

Let's face it, there's no way you can know what someone wants to do
with your drive.  IBM has had hardware keying in its drives for years
and years, and I know people who absolutely swear it's faster than
software keying could EVER be.

What's the problem with this?

For every access technique I've ever seen, there's an optimal and a
suboptimal series of requests.  There are a number of techniques which
boast near-even access times all the time, and a number of them which
produce access times which are near optimal for the hardware for
specific sequences.  None that I've seen can offer near optimal ALL
the time, for ALL given sequences.

You'd have to do that to get hardware to perform as well as software
would when software can know ahead of time how the accesses are going
to be done.  The application writer has the ability to examine how
accesses are going to be done and optimize the data layout based on
that knowledge.  The drive manufacturer does not.  Therefore, if the
software just happens to use the worst-case access sequence, it gets
terrible performance.

There are a hell of a lot of algorithm books out there which bend over
backwards trying to prove that you can't fool all of the people all of
the time, which is what you're wishing for if you think the hardware
designer can predict what everyone is going to want to do with the
hardware.

Happy hacking,

jim frost
saber software
jimf at saber.com