Path: utzoo!utgpu!attcan!uunet!microsoft!stevesc
From: stevesc@microsoft.UUCP (Steve Schonberger)
Newsgroups: comp.os.minix
Subject: Re: Big disks, big problems
Message-ID: <7917@microsoft.UUCP>
Date: 1 Oct 89 22:08:01 GMT
References: <3442@ast.cs.vu.nl>
Reply-To: stevesc@microsoft.UUCP (Steve Schonberger)
Organization: Microsoft Corp., Redmond WA
Lines: 40

Andy writes:
>In particular, since disk addresses are 16 bits, clearly no partition
>larger than 64M can work, but how about > 32M?

It is possible to work around this problem by using larger sectors.  I
know some of large-disk-DOS versions use this solution.  The 32M limit
assumes 512 byte sectors; addresses are unsigned, so with sectors of 1K
you get 64M without trouble.

>What about disks that are larger than 64M, but have a MINIX partition
>above 64M?  This probably implies longs somewhere, and I don't know if
>they are used properly.

Having several partitions of <32M has long been a large-disk-DOS trick.
It's quite simple, I think.

A third solution, using longer disk addresses, is difficult, and was
likely the root of a lot of the compatibility complaints about DOS 4
(but I don't know much about DOS, and even less about DOS 4, so I'm
no authority).).

A very elegant solution is that of Berkeley fast file system.  They use
sector clusters as the allocation unit.  Files that will fit whole into
a cluster only use the sectors they need; files that are larger than a
cluster are stored in whole clusters only.

Another interesting solution, used by Microsoft's announced and maybe
released OS2 High Performance File System is to use extents (starting
block, length pairs) as the allocation unit, rather than individual
blocks.  That would be highly inappropriate for Minix though, since it
is both very un-Unix-like and would require a great deal of rework to
implement.

-- 
	Steve Schonberger	microsoft!stevesc@uunet.uu.net
	"Working under pressure is the sugar that we crave" --A. Lamb
Since I've mentioned products made by the company I work for, I need
to emphasize my disclaimer.  I know very little about DOS.  I know
very little about the file system part of OS2.  Most important, I'm
not making official announcements.