Operationally, the assignment of devices is influenced by:
- The online/offline status of the device. Generally, to be allocated
to job steps, devices must be online. Exceptions are (1) when the
online test executive program (OLTEP) or a similar testing program is running
and (2) when teleprocessing devices are allocated. You can bring
offline devices online with the VARY command or in response to the
allocation recovery message, IEF238D.
- The MOUNT attribute. The MOUNT attribute, which applies only to tape or DASD devices,
is influenced by the MOUNT and UNLOAD
system commands, and, during initialization, by entries in the VATLSTxx parmlib member. Allocation requests
that can be satisfied by mounted devices are processed quickly and
without your intervention.
- The USE attribute. A parameter of the MOUNT command, the
USE attribute affects the type of data sets that can be allocated
on a tape or DASD volume. The USE attribute can also be set during
initialization by entries in the VATLSTxx member of parmlib. Having a proper mix of volumes
with various USE attributes reduces the amount of volume mounting.
The information from data definition (DD) statements determines
the input/output resources to assign to a job or job step and the
volumes that are required. If a requested volume is not mounted,
the system issues a mount message asking you to mount a specific volume
or scratch volume. If you
mount the wrong volume, the system finds out as soon as it reads the
volume label. The system unloads the volume and repeats the mount
message.
When you know that several jobs are going to need a volume, use
the MOUNT command to reserve that volume on a device. Allocation
processing is faster when the required volume is reserved rather than
removable. The system does not demount volumes reserved by a MOUNT
command until you issue an
UNLOAD command.
Never mount a blank tape volume unless specifically directed to do so because the
system scans the entire volume for a tape label and this scanning
wastes time. If an unlabeled tape is needed, write a tapemark to avoid unnecessary scanning. After you mount the tape volume and ready
the drive, the system reads the volume label. If an incorrect volume
is mounted, the system unloads the incorrect volume and repeats the
mounting message.
Note: - Occasionally, you receive two mount messages for the same volume,
one starting with IEF and the other with IEC. Treat the two messages
as though they were one. The second is a reminder.
- When referring to I/O devices in the devnum parameter
of system commands, use the unique 3-digit or 4-digit device number
for each device. You can precede the device number with a slash (/).
The slash is optional on many commands, but required for 4-digit
device numbers on some commands, such as MOUNT and START.
- Your installation can define symbolic group names of one to eight characters to be used by programmers
in data definition (DD) statements. The number of devices associated
with a symbolic name can range from one to the total number of devices
in your installation. The symbolic name allows the devices to be
grouped according to the attributes your installation considers significant.
Do not use these symbolic names in system commands.
- Make sure there are sufficient work volumes available to satisfy
requests for temporary data sets at peak loads. A shortage of work
volumes can cause the system to request additional scratch volumes.
Balance work volumes across channel
paths to increase system efficiency.