Every time you mount a tape there is a small but non-zero probability
that the tape will be physically or logically damaged by a drive (or by
an Operator or robot mishandling the tape). If that damage occurs near
the load point, you could loose everything on the tape. If you have a
process that mods repeatedly to the same tape volume over and over, you
are raising the probability of that tape being a victim. Since LOGREC
data would rarely be considered mission-critical, Murphy's Law has
probably seen fit to spare you.
In the case of non-critical data, there is nothing inherently wrong with
what you are doing, and there will be no problems as long as the tape
drive is functioning properly and the media doesn't have any problem
that results in incorrect file positioning. With today's tape
technology if would be very unlikely that a tape written with no fatal
errors would not be readable afterwards - unless some intervening
physical damage occurs to the media.
The minor inconvenience of an unlikely loss of data that you can live
without is not a big deal. For somewhat more important data it is a
better practice to keep input and output tapes totally separate (e.g.,
copy the input tape to a new output tape and append the new data to the
output tape, so that the process is repeatable as long as the input tape
doesn't become damaged. For really critical tape data, you would also
want to write two tapes at the same time, so you could recover from the
alternate if at some later time the primary tape were found damaged when
used as input.
I don't know if these recommendations are formally documented anywhere,
or are just one of those things that becomes reasonable after you've
been burned once.
Although DASD isn't subject to the same physical damage risk as tape,
you still have risk of job step failures or termination from other
causes, and use of DISP=MOD for DASD can make a failed job step very
difficult to rerun. In a production environment one needs to be very
cautious about use of MOD for either tape or DASD datasets because of
restart issues.
George Dranes wrote:
In respect to MOD, are you just referring to moding SMF data to tape? I've
successfully used mod (with ICEGENER) to tape for our LOGREC files for 17
years and have yet to have an issue with reading the tape. Actually we also
have a few other files we mod to tape which have also never failed. I now
understand the way to go for SMF is to use IFASMFD rather than ICEGENER to
move data around and not to use mod but with respect to using mod to tape
for other types of files I have yet to see an issue?? Am I just lucky or are
these just extremely isolated situations where mod to tape fails? I could have
very easily have missed it but has IBM made recommendations against using
mod for tapes? Thanks for all of the help!
--
Joel C. Ewing, Fort Smith, AR [EMAIL PROTECTED]
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