From: daniels@xstor.com (Daniel A. Segel)
Subject: Re: HUGE disks and minimum file size
Followup-To: comp.sys.mac.system
Organization: Storage Dimensions, Inc.
Date: Thu, 12 Nov 92 19:15:25 GMT

In article <RMF.92Nov9143202@chopin.cs.columbia.edu>,
rmf@chopin.cs.columbia.edu (Robert M. Fuhrer) wrote:
> I also think that the user should have some control over the min allocation
> clump size when partitioning. It's crazy to *have* to break a nice big drive
> into a whole bunch of small ones just to avoid wasting *large* amounts of
> space. As a better, long-range solution, a more reasonable allocation policy is
> in order. [Apple, are you listening?]
The operating system needs to store the number of logical blocks
(allocation blocks) in some form of variable. Apple chose a variable that
was only 16 bits wide, so you can only have up to 65,535 allocation blocks
on a hard drive. If the number of physical blocks (i.e. 512-byte or
1024-byte sectors) exceeds this amount, which will happen on any drive over
32MB, you need to allocate logical blocks equal to some multiple of
physical blocks (i.e. for a 64MB drive, make each logical block 2 physical
blocks, or 1024 bytes).

It would require a fairly big re-write of the operating system to suddenly
change over to a different system of keeping track of physical blocks on a
SCSI hard drive, but it could happen.....

> 
> Now, for a follow-up:
> 
> After the re-partitioning, I soon had a machine crash which damaged the boot
> partition. The system booted to a floppy-question-mark icon (no bootable
> volumne), and after booting from a floppy, said the drive was unreadable.
> 
> I have never before in 5 years of running this Mac II, 1 and 1/2 years of it
> with this drive, experienced a crash which took the hard drive with it.
I work in tech support for a hard drive company. We get upwards of 100
calls a week from people whose hard drives have crashed, and about 10-20%
of those calls involve a corrupted partition map. If you have software that
knows how to recreate the partition map, this isn't much of a problem.

Unfortunately, most software can't. In fact, most people don't know much
about partition maps, and when the partition map gets corrupted utilities
like Norton and MacTools tend to just say "there is a problem I can't fix"
and give up, when in fact there may be nothing wrong with the directories -
it's just that everybody is looking for them in the wrong place since the
partition map is pointing to the wrong place.

> Now, my question is this: is a partitioned drive on a Mac more susceptible to
> being damaged than a non-partitioned one? [I can think of several reasons why
> this might be the case, but I'm looking for empirical answers, not theoretical
> ones.]
There is a partition map on every Mac SCSI hard drive, and whether it has
one partition entry or twenty is irrelevant. The partition map can be
corrupted just like the directories, since it is just a bunch of data
sitting on the hard drive, albeit it is always in a specific location
(starting at block #1, going for upwards of 64 blocks, depending on the
formatting software used).

I guess one could say that the more entries there are in the partition map
the more likely that random garbage written to that part of the disk will
damage something, but I wouldn't say that makes single partition drives
safer...

One last thing: recovering data off of a crashed drive is always easier if
the utility used knows where to look for the directories; with more than
one partition there is a greater chance that the directories of the
additional partitions will not be locatable...

Daniel S.

___________________________________________________________________________
Daniel A. Segel, KD6NMT                      Macintosh Customer Engineering
Triumph Bonneville 650              Storage Dimensions, Inc. (408) 954-0710
daniels@xstor.com                     All opinions are of my own invention.