File Systems

[Richard Todd]

brian at fog.ann-arbor.mi.us (Brian S. Schang) writes:

>I understand that A/UX 2.0 is able to run two different file systems.
>I believe that one is the standard System V, and the other choice
>is the Berkeley Fast File System?  Is this correct?

Yep.

>Assuming the above is correct, what are the differences between the
>two?  What are the advantages of each?  I've gotten the impression
>that FFS is better, is this true?  Why?  Why would I not want to run
>it?  Are the two compatable and able to be mounted simultaneously?

  System V filesystem is basically the same as the old Version 7 filesystem 
(with only minor changes--block size 1K instead of 512 bytes, things like
that).  It shares the same limitation as V7 in that file names are limited
to 14 chars; also, the System V/V7 filesystem doesn't make much of an attempt
to avoid fragmentation.  The BSD FFS (Fast File System) attempts to do 
a better job at allocating files to avoid fragmentation; this means that the
disk head doesn't have to seek as much to find all the pieces of your file,
so file accesses are more efficient.  BSD filesystem also allows you to have
filenames with >14 chars (the limit is 255, I believe).  (For those curious
on just how the BSD FFS makes file I/O faster, the chapter on it in the book
_The Design and Implementation of the BSD4.3 Operating System_ by Leffler 
et al. will probably tell you more than you wanted to know :-)

  So, BSD FFS filesystems are more efficient (my not-terribly-rigorous
tests showed roughly a factor of 3 speed increase on my Miniscribe
9380S drive), and allow you to have longer filenames.  You can have
both System V and BSD- style filesystems mounted simultaneously with
no problem.  The only possiblereason you'd want to leave a filesystem
as SysV is if converting it would be a big hassle (e.g. it's a 100M+
filesystem and you don't have a tape drive to do dump/restore onto so
you gotta use floppies :-)

>Sorry about the abundance of vague questions.  I will be installing
>A/UX 2.0 on another drive soon (hopefully), and I would guess that
>that would be a good time to switch from Sys V to FFS if this move
>is desirable.
 
   It's an excellent time to do the switchover, since you're backing everything
up anyway (or at least you *should* be backing everything up :-).  
--
Richard Todd	rmtodd at uokmax.ecn.uoknor.edu  rmtodd at chinet.chi.il.us
	rmtodd at servalan.uucp