We have had a couple of clients that have verified it
made a very big and noticable improvement when they turned off this option
with CopyCat. And CopyCat is simply doing OPEN TYPE=J after increasing the
file-sequence number for the next volume, and the allocation of course has
RETAIN coded on it.
We also use OPENJ as you do, but we don't specify RETAIN. We do CLOSE
with LEAVE to leave the tape positioned at the end of the last file
written, except when we know we are writing the last file when we CLOSE
with DISP to rewind. Maybe it is the RETAIN that is biting you.
My test was on a 3590B.
My last response from IBM indicates that if the
volume supports MSNS(?) it will be used to verify the volume mounts; if the
drive does not support MSNS then the previous data sets trailer labels are
read to compare the label data to data saved in the UCB Tape Class extension
(which was saved during CLOSE processing of the previous file). So, if the
device you tested on did not support MSNS, it would only re-read the
previous tape labels (if my understanding of the email from IBM was
correct).
MSNS is undoubtedly Medium Sense, which detects the type of tape
(medium) mounted and probably returns the file position and maybe volser
(I don't have the reference handy). 3590 and above support Medium Sense.
But given IBM's comments, under what circumstances would they rewind the
tape???
--
Bruce A. Black
Senior Software Developer for FDR
Innovation Data Processing 973-890-7300
personal: [EMAIL PROTECTED]
sales info: [EMAIL PROTECTED]
tech support: [EMAIL PROTECTED]
web: www.innovationdp.fdr.com
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