Magnetic tape volumes: This parameter
indicates to the system whether the user is keeping an accurate block
count. (Maintain the block count in the DCBBLKCT field.) The system
ignores this parameter when the program is executing and the device
is not magnetic tape. The EOV and CLOSE functions can record the
block count in the IBM® standard
or ISO/ANSI standard trailer labels.
On input, the EOV and
CLOSE functions can compare the DCB block count with the block count
in the standard trailer labels to detect missing or duplicate blocks.
This is supported on reel and cartridge tapes.
On input or
output, the EOV and CLOSE functions can compare the DCB block count
with the block count calculated from tape cartridge subsystems to
detect missing or duplicate blocks. The system does this for any label
type and for unlabeled tapes. Refer to z/OS DFSMS Using Magnetic Tapes.
For
magnetic tape devices with reels (not cartridges), coding REPOS=Y
allows the dynamic device reconfiguration (DDR) function to move the
volume to another tape drive if the drive has a failure. With a cartridge, DDR can do this without your program maintaining block
count.
For magnetic tape devices with reels (not cartridges),
restart requires the block count in order to restore the tape to its
position at the time of the checkpoint. With a cartridge, restart
can do this without requiring that you maintain a block count.
The
system performs the following tasks:
- Y-The count is accurate.
- N-The block count is inaccurate.
If the operand is omitted, N is assumed.