[K12OSN] Got to rebulid -- how to take an image for checkpointing

[Chuck Kollars]

>With a (p)OS like Windows(TM), you need a
>"ghosting" program because the system has loads
>of secret binary data that keep it from working
>if you just copy the filesystem contents of the
>OS to another system.  Thankfully, Linux is part
>of the UNIX tradition and the entire system is
>portable.

(Background info: a "ghosting" program copies
raw sectors without understanding anything about
whatever OS they instantiate).

The inadequacy of a file-by-file approach is often
a moot point because disks are so large a
"ghosting" program is a practical necessity.
Anything that goes through the file system, when
faced with 80GB, can be rather slow.

Linux systems can't always be treated cavalierly
as just what's visible from `ls`.  True, Windows
is seldom backed up simply as a large collection
of files; but the reason is there are few really
adequate backup programs, not because it's 
theoretically impossible.

So what exactly is going on?  There are several
"problematic" areas in viewing any OS through the
lens of its file system:

1) "Hidden" and "system" file attributes are
particularly problematic.  But this problem
doesn't just affect Windows.  In Linux files whose
name starts with a "." are conventionally not
displayed.  Ever done a `cp` only to discover none
of the "dot" files were copied?

2) Several OSs have alternate portions of files
that aren't normally visible.  Mac's have the
"resource fork"; Windows has "streams".  These are
visible through standard (if little-used) system
calls; there's no reason a really good backup
program couldn't handle them correctly.

3) In most modern OSs there can be "metadata"
associated with each file.  Even Linux has this
option, which unfortunately most of its programs
don't handle  correctly.  Have you ever backed up
a file system that uses ACLs (Access Control
Lists) with `tar`, restored it elsewhere, and
wondered what happened to all your ACLs?

4) Some applications (think "AD") are paranoid
about security but don't support adequate recovery
procedures.  Consider an application that changes
its hashes every fifteen minutes, embeds
timestamps in its data, and doesn't provide a way
to resynchronize.  It's almost impossible to back
up, not only as a collection of files but even as
a collection of raw sectors (which is how 
"ghosting" tools would see it).

So yes doing a naive file-by-file copy often 
doesn't do what you want it to, especially with
Windows. But no there's no need to postulate
secret binary data to explain why.  

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