Path: utzoo!utgpu!water!watmath!clyde!rutgers!sri-spam!ames!pasteur!cory.Berkeley.EDU!dillon
From: dillon@cory.Berkeley.EDU (Matt Dillon)
Newsgroups: comp.sys.amiga
Subject: Re: Semicoherent flame about Amigados.
Message-ID: <687@pasteur.Berkeley.Edu>
Date: 11 Feb 88 19:30:07 GMT
References: <644@nuchat.UUCP>
Sender: news@pasteur.Berkeley.Edu
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[This is Bryce Nesbitt- Please don't try to mail any mail intended for
me... things are broken at the moment]

In article <644@nuchat.UUCP> peter@nuchat.UUCP (Peter da Silva) writes:
>Time for my periodic semicoherent flame about Amigados.
>
>...Almost every disk error means you lose an
>entire track, thanks to the 1 sector per track concept...

No, no, no.  Cause and effect is all messed up here.  Your disk errors have
a cause, but I maintain that it is not AmigaDOS proper that is causing
them.  The trackdisk.device is the responsible entity.	I'll get to that in
a moment, but first:

IF YOU ARE HAVING CONSTANT READ ERRORS, EITHER YOUR DISKS OR DRIVES ARE
DEFECTIVE, AND MUST BE FIXED.

Ok.  When trackdisk has a buffer of partially good, and partially bad,
sectors, it often will loose the entire track.	This has nothing to do with
the "one track per revolution" disk format scheme used.  Instead trackdisk
simply does not have enough brains to see that there is a problem.

Trackdisk should have a bunch of options, including writing out the data
many times so that the bad spot on the disk matches the already bad sector
or the tail gap.  This saves whatever good data that was on the track.

In general, trackdisk leaves a lot of tricks untried as far as dealing with
cripple disks.	The Slow File System (SFS) has a lot of redundancy that
compensates.  Many of the OS utilities swing the other way.  For example,
FORMAT will retry many times.  Here you have this disk that format had to
retry 5 times, and you want to write data on it?  The least format could do
is tell you that the disk might not be perfect.

Bad block mapping within trackdisk probably won't work under SFS, but I
could be surprised.


PS: I'm convinced that a disk that is bad during manufacture is never
actually thrown away... it just gets passed to the next lower/cheaper disk
distributor :-).

PPS: Random thought on AmigaDOS design:  The checksum and redundancy area
of AmigaDOS is put at the start of the sector.	This makes sector-by-sector
DMA inconvenient.  If the same data was at the END of the block, DMA would
be different.  You could DMA the entire block in, past it's local end.
Check it while the controller perpares the next sector.  Now overwrite the
first block's redundant area with the next sector.  Only the last sector
would need a special case.


|\_/|  . ACK!, NAK!, EOT!, SOH!
{o o} .     Bryce Nesbitt
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  U	    Don't send mail... it won't get here.