Path: utzoo!attcan!uunet!husc6!bloom-beacon!mit-eddie!rutgers!ucsd!ucsdhub!hp-sdd!hplabs!hp-pcd!hpcvca!charles
From: charles@hpcvca.HP.COM (Charles Brown)
Newsgroups: comp.sys.amiga
Subject: Re: Request to Commodore (Bad Blocks)
Message-ID: <5660016@hpcvca.HP.COM>
Date: 15 Sep 88 14:42:35 GMT
References: <8891@cup.portal.com>
Organization: Hewlett-Packard Co., Corvallis, Oregon
Lines: 36

>>	If you always start a track read at the beginning of a track
>>	(as written) you will on the average have a 1/2 rotation
>>	latency waiting for the disk to turn to the start of that
>>	track.  So decoupling the index hole from the the start of
>>	track did not speed up reads AT ALL.
> But you don't always start a read at the beginning of a track.
> Trackdisk.device starts reading wherever the head is, and reads one
> full revolution plus enough extra to make sure it gets at least one
> intact copy of the sector it started reading on.  So the total track
> read time should be constant:
> 	1 rotation + 1 sector time + 1 index gap time.
> 
> Since the gap varies due to variation in disk speed, etc., you have to
> allow some slop.  I'd roughly estimate the result as being 1.1 rotations,
> not the 1.5 you came up with, so it does save a fair amount of time.
> 					-=] Ford [=-

Your argument only applys to reads.  I am saying "don't change the way
reads are done".  So let me work thru the numbers with your correction:

> The following numbers represent time in terms of track rotations:
> 		  write	+ read (weighed)
>  index coupled:   1.5	+ 5*1.5		= 9.0
>  index decoupled: 1	+ 5*1.5		= 8.5
> Now the advantage of decoupled index drops to 8.5/9.0=0.94 or 6%
becomes:

The following numbers represent time in terms of track rotations:
		  write	+ read (weighed)
 index coupled:   1.5	+ 5*1.1		= 7.0
 index decoupled: 1	+ 5*1.1		= 6.5
Now the advantage of decoupled index drops to 6.5/7.0=0.93 or 7%

So we gave up track reliability for at best a 7% increase in overall
floppy speed.  That strikes STILL me as a bad trade.
	Charles Brown	charles%hpcvca@hplabs.hp.com