Making a bootable backup (hard)disk... how?

Matthew Seaman m.seaman at
Mon Jun 11 07:29:10 UTC 2012

On 10/06/2012 23:14, Ronald F. Guilmette wrote:
> Well, nevermind about that.  I get the general idea, i.e. that dumping
> at level N causes dumping of everything that has changed since the last
> dump at level N-1.

Not quite.  A dump at level N includes everything that changed since the
most recent dump at level M where M<N.

So if you follow a level 0 dump immediately by a level 2 dump, the level
2 dump contains all the changes since the level 0 dump.  Which is
exactly what you would expect.

Where this gets useful is if you consider what happens if the next dump
after that is a level 1 [+].  You get the difference between the
original level 0 dump and the current filesystem -- so you only need the
level 0 and level 1 tapes[*] to do a restore.

However if the third dump was a level 3, you'ld get the differences
between the level 2 dump and the current filesystem, so you'ld need all
of the level 0, level 2 and level 3 tapes to do a restore.  It's a
toss-up between economising on the quantity of tape used and the hassle
factor in doing restores.



[+] Actually level 2 or above.  A dump level sequence of 0, 2, 2 has
the same effect as 0, 2, 1.

[*] For values of "tape" that include all sorts of different backup media.

Dr Matthew J Seaman MA, D.Phil.                   7 Priory Courtyard
                                                  Flat 3
PGP:     Ramsgate
JID: matthew at               Kent, CT11 9PW

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