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From: tom@afthree.as.arizona.edu (Thomas J. Trebisky)
Newsgroups: comp.sys.att
Subject: Re: Drive type in 3b1
Keywords: att 3b1 hard drives
Message-ID: <1240@organpipe.UUCP>
Date: 20 Mar 91 17:14:09 GMT
References: <107@kbvan.com> <2130@public.BTR.COM> <1991Mar19.180851.27460@oswego.Oswego.EDU>
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Distribution: usa
Organization: University of Arizona, Tucson, AZ
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In article <1991Mar19.180851.27460@oswego.Oswego.EDU> ostroff@oswego.Oswego.EDU (Boyd Ostroff) writes:
>
>The spare sector concept is correct, but the numbers are wrong.  Due to the
>fact that the 3B1's disk controller can only handle a certain number of
>sectors (I forget the details), there are "spares" on each track which are
>used for mapping bad blocks.  The full, formatted capacity of the Miniscribe
>drive is actually 71MB, but the unix-pc can only utilize 67MB.

Actually the disk controller (hardware) is perfectly happy with 17 sectors
per track.  The 16 sector per track with sector 17 reserved as a spare is
purely a software convention handled (presumably/hopefully) in the kernel
device driver.  The main motivation is to avoid long (and thus slow) seeks
going to fetch replacement blocks when bad sectors are encountered.

I took exactly the opposite approach writing my device driver for the miniframe.
I use the last cylinder of the disk for spare blocks and suffer with the
long seeks (but my typical 40M disk only has maybe 6 or so bad blocks at
this time).  This gives me 2M of additional capacity -- but the other trade off
is that 17 is an ugly number -- filesystem blocks have to wrap around from
one track to the next (I am using a 4K/512 BSD filesystem). You pay your money
and you take your lumps -- I opted to loose some speed and gain capacity.
However in the swap area, I will treat the disk as if it had 16 s/t and
ignore all the last sectors on each track -- speed is of the essence there.
--
	Tom Trebisky	ttrebisky@as.arizona.edu	(Internet)
	Steward Observatory	University of Arizona	Tucson, Arizona