Re: What is a "backup level" ?

On Thu, Feb 20, 2003 at 11:52:18AM +0100, Gilles Bourcy enlightened us:
> Hello
> 
> First of all, thanks to Joshua Baker-LePain for his answer. It helped me 
> well; now
> Amanda is correctly configured, and runs properly.
> 
> My problem now is that I don't fully understand the notion of "backup 
> level". And it seems
> important to correctly understand it for amrecover, or more simply for a 
> general understanding
> of how Amanda works.
> 
> Can you help me ?
> 

Simply put, a backup level is the files that have changed since the last
level. A level 0 is a full backup of every file. A level 1 is every file
that has changed since that level 0. The way amanda works, it will only bump
up to the next level when a certain amount of space can be saved by doing
so. So in a typical week, you might have something that looks like this:

Day 1: Level 0 - 2,000,000 kB (Full backup)
Day 2: Level 1 -    50,000 kB (Contains changes since Day 1)
Day 3: Level 1 -    75,000 kB (Contains everything from Day 2 plus new stuff)
Day 4: Level 1 -   150,000 kB (Contains Day 1, Day 2 and new stuff)
Day 5: Level 2 -    10,000 kB (Contains changes since Day 4)
Day 6: Level 2 -   300,000 kB (Contains Day 4 and new stuff)
Day 7: Level 3 -    15,000 kB (Contains changes since Day 6)

So, to completely restore this directory from tape, you start with the level
0. Then you lay the *latest* level 1 over top of it, which would be Day 4.
Then the latest level 2, 3, 4, and so on. In this case, we would need 4
tapes to be able to restore a directory. 

Fortunately amanda can keep track of files for you (see the index option in
amanda.conf), so that you only need one tape for a specific file. amadmin
can tell you "The last time you backed up file foo was on Day 6", so you can
restore it from that tape and know it was the latest version since.

Hopefully that makes sense...I know it took a little while for me to be able
to wrap my head around that notion as well. :-)

Matt

-- 
Matt Hyclak
Department of Mathematics
Ohio University
(740) 593-1263
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