Re: Is tape spanning documented anywhere?

On Tue, 13 Jun 2006 at 3:05pm, Paul Bijnens wrote

On 2006-06-13 12:55, Toralf Lund wrote:

It could also help the current minor problem that taping starts only
when the DLE is completely dumped to holdingdisk.
The current implementation also assumes the tape chunks follow
sequentially on the tape.  This is not strictly necessary either.

Allowing tape-chunks to be interspersed with chunks from other DLE's
together with multi-run taping... Wow, that would make Amanda really
one of the best free backup programs!

Again, let the curmudgeon step in here. One of the initial designprinciples of amanda was the ability to get your data off the tapes with*no* amanda tools -- mt, dd, and tar or restore were all that was needed.Tape spanning as implemented has already broken that, requiringamfetchdump to reassemble spanned DLEs...

I think. I honestly don't know how badly the principle has been broken.Can one simply cat the 2 (or more) spanned images together (minus someheader info perhaps) and get the whole image back?

But I do know that interspersing tape chunks from multiple DLEs wouldabsolutely destroy any hopes of getting your data off the tapes withoutamanda's tools *and* record keeping. With live CDs so prevelant thesedays, keeping copies of the amanda tools around is dead easy. But,IMNSHO, losing the ability to get your data off the tapes if you lose youramanda database is unacceptable.

Only a small step further and you can use the gnutar option
--record-number (show record number within archive of a particular
file) making it possible to restore from only a few tape-chunks,
instead of feeding the complete 300 Gbyte image to tar, to extract
only one file, which happens to be at the end of the image by
murphy's law anyway.  :-)

This *is* something that would be nice to implement. But I'd like to seeit implemented in a way that makes it optional. Amanda holds onto recordnumbers. But, again, if you lose your amanda database, the whole tarballwould still be there for you to recover and feed to tar directly.


--
Joshua Baker-LePain
Department of Biomedical Engineering
Duke University