Tape Imaging

Stan Sieler sieler at allegro.com
Thu Aug 11 16:08:29 CDT 2016

Joining the list of "my format" posts ...

Mine also records retry information (because MPE on the HP 3000
optionally reports if a retry was done to get a successful tape read),
and setmarks (which differ from EOFs), as well as error information.
(That retry information is important ... it could indicate a silent loss
of information.)

But, I must admit...it didn't occur to me to store metadata like
a photo of the tape, etc.  Nice!

When copying / archiving tapes ...

One important thing to do, depending upon your operating system
and tape drive characteristics of course, is to issue read requests
for a few bytes more than you expect ... because with some OSs and
some kinds of drives, if you ask for X bytes and the record has more than X,
you'll quietly get X and the rest will be discarded.   (That came up in a
court case where I was an expert witness ... an alleged 'expert' had
copied a 9-track tape (badly) and lost data because the records were larger
he expected, and his copying tool didn't have that simple safeguard in it.)

A second thing is to be somewhat aggressive in reading the 'end' of the
The backup tapes I frequently encounter supposedly end with two EOFs
in a row ... except for a few that happen to have extra data past that
point :)
(Of course, with 9 track tapes, you run the risk of going off the end!)

(Yes, that begs the question...if you're archiving a tar tape ... do you
the data past the first EOF?  (Which could be part of a prior (and longer)
or something else.))  (And then there are the people who put tar after tar
after tar on the same tape :)


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