[Info-vax] strange backup behavior

[glen herrmannsfeldt]

AEF <spamsink2001 at yahoo.com> wrote:
(snip)
 
< I have a TZ88 and it seems to take a long time to get to the end of a
< long save set. Have you checked? Why would the drive have 128 heads?
< Sounds pretty expensive when one would do! (Actually, it has two. See
< below from the TZ88 manual.
 
< 1.4 Reading and Writing Data
< The TZ88 Model 20/40 GB DLT Cartridge Tape Drive writes 64 pairs of
< tracks ? 128
< tracks total ? on the CompacTape IV tape. The drive reads and writes
< data in a two-track
< parallel, serpentine fashion, traveling the entire length of tape on
< two tracks. The drive then
< steps the head, reverses tape direction, and continues to read/write
< on the next two tracks, repeating
< this same process for a total of 64 times per tape.

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.

For LTO, an EEPROM is used, for DLT data written by the drive
before BOT.   In unix, one can do 

dd fsf 10 

to skip forward 10 tape marks.  If the software/hardware can do it,
it will be done.

-- glen