[Info-vax] strange backup behavior

[glen herrmannsfeldt]

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

>> 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.

> Nice trick! But I thought you can't write to a tape before its logical
> EOT. I guess with this, you can. I suppose it leaves a large gap
> between the special area and the normal data storage.

You can't, but the drive can.  DDS drives record usage information,
I don't know about tape mark information, at the beginning.
Also, DDS drives refuse to read anything past EOT.

In the 9 track days, it was possible to ignore EOT and continue
reading.  If one wrote a small file at the beginning with a new
EOT on an existing tape, one could still read old data after that file.

With DDS the only way around that is to start writing, write over
the old EOT, then power down the drive before it can write a new EOT.
(With careful timing before you overwrite too much of the old data.)
> 
>> I presume that this wasn't done for DLT-I, but I don't know which
>> version it was implmented on. ?
> 
> Not on DLT-4 -- at least not on mine!

Maybe it is only SDLT.

-- glen