Explaining snapshots (for backup)

[rhkramer@gmail.com]

I'm not really clear on the concept of a snapshot (for backup) -- I've done a 
little googling but haven't found an explanation that "satisfies" me.

In this email I want to hyptothesize on what a snapshot might be in the hope 
that others can correct / amplify it when I go wrong.

Starting from a beginning, I suppose I could copy the entire contents of 
whatever I wanted to make a snapshot of (by any of a variety of tools -- dd, 
cp, ...) and call that a snapshot, although the more common name for it would 
be a "full backup".

Later, I could copy any files (as files) that have changed since the date I made 
the full backup and call that a snapshot.  (I might use find to identify the 
files that have changed, and then any of a variety of tools to actually copy 
them somewhere (and call that a shapshot).)

I suppose it wouild be possible to do something like this at a block level, 
that is copying only blocks of a file which have changed.  (Not sure which 
commands might help me do that, but I don't think I'd really be interested in 
doing that.)

I also hear (i.e., read) statements from which I infer that some snapshots 
included only the metadata of the files (or blocks???), but I'm not sure of the 
value of either of those to me -- how can you reconstruct a possibly missing 
file (or block) from the metadata? 

Thanks!

-- 
rhk

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