Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!ucbvax!hplabs!hpfcdc!hpfcmr!baker
From: baker@hpfcmr.HP.COM (Jim Baker)
Newsgroups: comp.databases
Subject: Re: Informix Trashed My 386 ...
Message-ID: <620001@hpfcmr.HP.COM>
Date: 22 Feb 89 20:33:34 GMT
References: <9662@ihlpb.ATT.COM>
Organization: HP Fort Collins, CO
Lines: 20

> What is a "magic number" and how can it be fixed when MIA?
A magic number is a bit pattern placed in a specific location to authenticate
the data structures/objects. For instance, a.out executables contain a magic
number to define the type of executable: demand paged MC68020. 
See the man page file(1).

The data structure that defines the format of the BSD filesystem
(super-block) contains a magic number to assure that some random
bits on the disk aren't masquerading as the super-block and in your
case to trap instances of super-block corruption. The BSD super-block is
replicated in several locations on the disk so that in the event the
primary super-block gets trashed, another structure can be used to 
recover the filesystem. The block offset of the spare super-blocks
can be given at file system check time to salvage (if possible) the
file system. Spare block locations are listed when the file system is
built and should be recorded & saved (newfs).

Good luck,
Jim Baker
Hewlett-Packard