using backup with 1/4 inch tape media

[Lanfranco Albani]

>Second, I did several backups to the same tape cartridge (w/o a tape
>label).  Everything finished correctly, so I did a tcopy on the tape
>and found out that my 60 MB cartridge had almost 112 MB on it!
>
>Is backup ignoring the hardware and always writing high density?  This
>would explain both problems above.

I believe there is something more to it... the AIX filesystem structure
is tricky, but what I _think_ is happening is that full 8-Kbytes blocks
are being used, thus with an average of 4 Kbytes wasted per file; df is
reporting on a full-block basis; and tape backup is NOT copying the
extra per-file 'slack', only the "meat" of each file (I'm not sure what
both df and tape backup are doing with holey files, but I would guess
them to be more in-sync in this respect).

I had a filesystem occupying 190 megabytes, and its backup fitted inside
a single DC6150 150-Mbytes tape; and I don't even know if that tape was
actually full... no problems re-reading it, anyway!

Well, at least this is saving us tapes!

-- 
Lanfranco Albani - CAD.LAB s.p.a., v. Stalingrado 45, Bologna, Italia
Email: (work:) bob at cadlab.sublink.org, (home:) bob at allan.sublink.org
Phone: (work:) ++39 (51) 371099, (home:) ++39 (51) 727372; 
Fax: ++39 (51) 366964 (work only), Fidonet: 2:332/401.4 (home only).