[Info-vax] strange backup behavior

[glen herrmannsfeldt]

AEF <spamsink2001 at yahoo.com> wrote:
(snip, I wrote)

<> Yes. ?But given a command to skip to a specific tape mark, the
<> drive knows which track and approximately where on the tape that
<> tape mark is. ?The heads are moved to the appropriate track, and
<> with an average of one half a pass through the tape the spot is found.
 
< How does it know?
(snip)
 
< I don't think we're talking about the same thing. If you are putting
< multiple save sets on one tape, you don't need to rewind or skip
< unless you use /verify, which the OP is not.

If you don't rewind, then there is no problem.  If you do then you
want to be able to find the EOT fast.
 
< The only time one would need to do this is to find the EOT mark on a
< tape that already has save sets on it when you put it in the drive.
< Tell me how the tape drive is going to know what tracks the tape marks
< are on when I go to my library (a drawer in a cabinet), pick some tape
< at random, and put it in the drive.

I know if for LTO better than DLT.  As I understand it, when the drive
rewinds the tape it then writes the known tape mark positions onto
a special region at the beginning of the tape.  (Before the actual
data storage starts.)  Then when loading a tape, this information is
read back.  

Given a command to seek to a specific tape mark, including EOT,
the drive can go directly to the appropriate track and approximate
tape position.  Then search for the actual tape mark on the tape.

I presume that this wasn't done for DLT-I, but I don't know which
version it was implmented on.  

-- glen