Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!wuarchive!gem.mps.ohio-state.edu!ginosko!uunet!mcsun!hp4nl!star.cs.vu.nl!ast
From: ast@cs.vu.nl (Andy Tanenbaum)
Newsgroups: comp.os.minix
Subject: Re: switching file systems
Message-ID: <3544@ast.cs.vu.nl>
Date: 6 Oct 89 12:05:24 GMT
References: <24987@louie.udel.EDU> <2453@ucsfcca.ucsf.edu> <1989Oct5.181148.8229@utzoo.uucp>
Reply-To: ast@cs.vu.nl (Andy Tanenbaum)
Organization: VU Informatica, Amsterdam
Lines: 30

In article <1989Oct5.181148.8229@utzoo.uucp> henry@utzoo.uucp (Henry Spencer) writes:
>Filesystem switches have many uses other than filesystem conversions.
>And that "one time minor task" is one-time and minor only if it works
>perfectly the first time.  

My hope is that things will be reasonably tested out before the definitive
posting.  At first I thought this was not a practical idea, but I have had
some second thoughts.  What I intend to do is to have an in-core inode
structure that is reasonably decoupled from the bits on the disk. For
example, in the incore inode structure, a disk address will be 32 bits,
even though on the disk it is only 24 bits.  This doesn't allow larger
disks, but if makes dealing with disk addresses easier.

The real reason for decoupling, however, is to make the IBM and Atari
disks exactly the same (little endian).  On an Atari, one would read in
an inode, fumble with it a little, and get a proper in-core inode structure
with the same 32-bit addresses as on the PC.  I think that only 6 routines
would be needed:
    read and convert an inode
    deconvert and write an inode
    read and convert a bit map
    deconvert and write a bit map
    read and convert a superblock
    deconvert and write a superblock

I don't think these routines will be more than a page or two.  Anyway,
when the time comes, I'll look at it.  Does anybody object to having the PC
and Atari disks be medium compatible?

Andy Tanenbaum (ast@cs.vu.nl)