Device allocation is the assignment of input/output devices and volumes to job steps. Requests for device allocation come from data definition (DD) statements and dynamic device allocation requests.
Installation programs that run on the system can specify dynamic device allocation/unallocation requests.
To control the amount of work needed for device allocation, you might want to restrict device allocation requests. You can define default values for allocation processing in ALLOCxx of the parmlib concatenation. ALLOCxx allows your installation to specify space, data set, and other allocation parameters for dynamic allocation requests. For more information about ALLOCxx, see z/OS MVS Initialization and Tuning Reference.
You can specify installation exits that get control whenever an allocation request occurs to perform further processing. In these exits, you can cancel the job that is making the request or satisfy the allocation request without having an operator perform actions like mounting volumes or varying devices on or offline. For more information about allocation exits, see z/OS MVS Installation Exits.
To control device allocation requests from DD statements, you might restrict each of the forms of input for these statements (for example, by holding the reader, or by setting a maximum LOGON count). However, because they originate within executing programs, you cannot control dynamic device allocation/unallocation requests.
Use VATLSTxx in the parmlib concatenation to control how to mount volumes for an installation. Based on the values you set in VATLSTxx, operators can issue MVS™ MOUNT and UNLOAD commands to mount or unload volumes efficiently. See Specifying shared DASD mount characteristics for a description of mount characteristics.
At IPL time or whenever a VARY command is issued, the system uses the VATLSTxx entries that you have specified. VATLSTxx helps reduce the amount of volume mounting so the system can process allocation requests for mounted devices quickly. Allocation processing is also faster when you define volumes as reserved rather than removable. For information on allocating devices in a multisystem that shares DASD, see Controlling shared DASD. For more information using VATLSTxx, see z/OS MVS Initialization and Tuning Reference.
If a requested volume is not mounted, the system issues a mount message asking the operator to mount a specific volume or scratch volume. If the operator mounts 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.
If your system uses automatic volume recognition (AVR), operators can mount labeled volumes on unused drives not managed by JES3. The system recognizes these volumes and assigns the drives to later job steps as required.
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. Operators can bring offline devices online with the VARY command or in response to the allocation recovery message, IEF238D.
Operators can also specify that a pending offline device is eligible for allocation through their response to message IEF238D.