Very Computer

We have an external DAT drive 1.3MB (+ compression) that I can use for
backing up my Linux box.

My question is, how do you make multiple backups on the same tape in such
a way you don't have to redo everything when/if you want to update a
session different from the last ? Is this possible at all ? Or do you have
to either rebackup everything next time or just replace the last session ?

--

Macintosh / Unix / Win (aaargghh) programmer -  System Administrator
Microsoft philosophy since 1984: Why innovate when you can immitate

Use /dev/n<device> instead of /dev/<device> for all your work.
the 'n' is for non-rewinding, I believe.
man 'mt' for instructions on how to move around on the tape.

i.e.
mt -f /dev/nrmt0 fsf X
or
mt -f /dev/nrmt0 eom

tar cpvf /dev/nrmt0 ./work_stuff

Quote:>My question is, how do you make multiple backups on the same tape in such
>a way you don't have to redo everything when/if you want to update a

That will forward the tape so that you can write the next archive.

For backing up, I suggest you do a full backup first (which it
looks like you did) then make incrementals until you run out of
tape. I did that (in DOS) for a long time - I had two tapes, but
had a lot of room on the second tape after a backup, so I did
a daily backup on the second tape until there wasn't room left, and
then I did a full backup, starting the cycle over.

The easiest way (using tar) to do incrementals is to touch a file
such as /etc/last.backup, at the end of a full backup. This file
doesn't have to even take up any disk space, but it will serve
as an indication of when the last backup was done. Then just
find all files newer than it:

$ find / -newer /etc/last.backup

Then use that as arguments to tar:

tar cf /dev/rmt0 `find / -newer /etc/last.backup`

And when complete, touch /etc/last.backup. As long as you remember to
always retouch this file after a backup, you won't have to backup any
files that haven't changed.

Careful...

Dates won't get changed many times when decompressing a tar archive,
moving files, etc!

--
Joel Maslak

Today's dreams WILL become tomorrow's realities!

Depending on tape drive and kernel options, it may not be possible to
reliably position your tape repeatedly at end-of-data for writing
an additional file. Worse, it may work MOST of the time.

I tried it with one particular combination. It worked about 80% of the
time. Good enough for backups?

So before you commit to using such a scheme, run some tests. Do a
bunch of identifiable backups, then try checking to see if what you
read back from the tape is what you expect.