Path: utzoo!utgpu!attcan!uunet!cbmvax!rutgers!mailrus!uflorida!gatech!linus!mbunix!eachus
From: eachus@mitre-bedford.ARPA (Robert Eachus)
Newsgroups: comp.sys.amiga
Subject: Re: Request to Commodore (Bad Blocks)
Summary: I think you have something here...
Keywords: trackdisk.device
Message-ID: <40244@linus.UUCP>
Date: 23 Sep 88 16:05:01 GMT
References: <8891@cup.portal.com> <5660018@hpcvca.HP.COM>
Sender: news@linus.UUCP
Reply-To: eachus@mitre-bedford.arpa (Robert I. Eachus)
Followup-To: comp.sys.amiga.tech
Organization: The MITRE Corporation, Bedford, Mass.
Lines: 58


     Suddenly a light bulb appears over my head, switched on and
shining brightly.  Charles has had a brilliant idea, and like most
great ideas it only seems simple IF you understand it.

> -------------------------------------------------------------------
> * DO NOT CHANGE THE WAY YOU READ.  ONLY CHANGE THE WAY YOU WRITE. *
> -------------------------------------------------------------------
>Therefore READS TAKE THE SAME AMOUNT OF TIME whether the writes are
>decoupled from the index pulse or not.

      This is the  key insight.  Writes  can be coupled  to the  index
pulse, and  disks written by a  "smarter" trackdisk.device can be read
(and written  to) by  an   old  trackdisk.device with  NO compaibility
problems.

>We gave up track reliability for at best a 7% increase in overall
>floppy speed.

     Not quite, Charles, adding  track reliablity won't cost even that
much. Right now, trackdisk.device must rewrite an entire track even if
only  one sector is   changed, and more  important  must  read a track
completely to write one  sector.   If a "smart" trackdisk.device knows
where sectors  are located,  it  can do  single  sector writes  in  an
average of 0.7 rotations,  instead of 2.2.  For compatibility  reasons
it should not do this on non-FFS floppies, but if a floppy is FFS then
there is no reason  to allow for  the "old style"  writes.  This means
that ALL  accesses would be as  fast or  faster except  formatting and
full track writes.

>It amazes me how easy it is to respond to a note without reading it.

     Most of  us (including  Ford)  try  hard to UNDERSTAND   what the
author had in mind before responding.  In this case, I think  that the
net traffic should be taken as evidence that some ideas  are very hard
to explain.  In fact,  since  many people probaably  don't  understand
yet, let me take another try.

     The key insight from Charles is that the model of the medium need
not be the  same when reading  as  when writing.   When  reading it is
sometimes useful to ignore some of the rules which must be obeyed when
writing.  In  this case  the hang-up that  most of us  have is that if
tracks begin at the beginning they must be read from the beginning.

     Okay who is  going to  be  the first   on  his  block to  hack up
trackdisk.device and try this for real? And please, Commodore, require
that FFS sectors be keyed off of  the index so that the "incompatible"
fast single sector writes can be supported, even if the implementation
of such writes has  to  wait for 1.5.   Note that this  feature really
will have a significant impact on performance, since it  will often be
used to update directory entries.


					Robert I. Eachus

with STANDARD_DISCLAIMER;
use  STANDARD_DISCLAIMER;
function MESSAGE (TEXT: in CLEVER_IDEAS) return BETTER_IDEAS is...