Using INZTAP writes a new volume label on the tape "followed by 2 tape
marks." <-- internal tape format stuff.
The 2 tape marks indicate to the operating system that there's nothing
else on the tape. Yes, the old data is still there, but the OS stops
reading at the 2nd tape mark in a row.
When you add data to the tape the 2 tape marks are written over with
header labels, assorted data, and single tape marks. At the end of data
on the tape the system writes end of volume 'headers' followed by 2 tape
marks.
The 2 tape marks indicate end-of-data. Nothing more on the tape is
read.
Check out
http://publib.boulder.ibm.com/infocenter/iseries/v5r4/index.jsp?topic=/r
zatb/hdr1d.htm for a nice explanation and diagram of what's on a tape.
The example is from a reel-to-reel tape, but I'm pretty sure the
concepts and sequence of things on a tape is still valid.
Wa-a-a-a-ay back in the System/3 days we had a recovery tool that would
read -past- the double-tape-mark that signified end-of-data and explore
what was lurking on the tape from previous uses. If you had only
initialized the tape, chances were good that you could still read your
files. Sort of like dropping something into the trash can on your PC--
the data is still there, and if you haven't re-used the space you can
un-delete the file.
That's why you need to use CLEAR(*YES) if you want to sell your used
tapes to someone else!
Paul E Musselman
midrange.com
