[clug] Can I re-record just the first record of a tape?

Brett Worth brettw at cray.com
Mon Jul 28 22:47:16 EST 2003

On Mon, 28 Jul 2003, Kim Holburn wrote:

> Maybe if you write two (small) files first then your archive.  Then write the first file again.  That way even if rewriting the first file overwrites the second file some way the backup which is the third file on the tape should be OK, bearing in mind that a "file" on tape has a minimum size (2Meg?).

I seem to remember that an EOF mark is written twice at the end of a write
to denote the last thing written. e.g.


When doing mt fsf sommands you are moving to the point just after the
filemark.  Thats why you can append.  You will get an error when you try to
read past the double file mark.  If I rewrote the above tape (even with a
shorter block) I'd get:


You'd need to convince the tape drive not to write the second EOF mark 
somehow.  You probably cant.

A while ago Drake Deitrich posted a CDR backup utility which used a
staging area to assemble enough files to fill a CDR before writing.
If you did something like this with your tapes you could write whatever
you liked at the start of each tape.  All you need is a bit of spare disk


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