Dynamic Allocation

Dynamic allocation obtains resources as needed. For an explanation on how to use and specify dynamic allocation, see z/OS MVS Programming: Authorized Assembler Services Guide.

For each invocation of dynamic allocation on a global or local processor, JES3 identifies those dynamic allocations that are a single request; that is, the request has a single SIOT/JFCB pair. The following requirements must also be met:

Request is for a single unit (equivalent to specifying UNIT=nnnnnnnn or UNIT=(nnnnnnnn,1) on the DD statement)
Request is for a specific volume (equivalent to specifying VOL=SER=serial number on the DD statement)
Request is for a volume that is permanently-resident or reserved on a real DASD.

JES3 manages all requests for SMS-managed data sets unless SMSSETUP=NO is specified on the SETPARAM initialization statement. However, JES3 is not aware of specific units for SMS-managed data sets. JES3 treats requests for these data sets as single requests and sends them through dynamic allocation - fast path. All dynamic allocation requests for SMS-managed data sets that require more than one SIOT, such as GDG-all requests, will be sent through MDS for processing instead of dynamic allocation - fast path.

When JES3 identifies a request that meets the requirements described, it sends the request to the DYNAL DSP.

Those requests which do not meet the requirements are passed to MDS for dynamic allocation under the SETUP FCT.

Use the ALWIO parameter on the SETPARAM statement to specify the current number of asynchronous I/O requests that can be processed at the same time by the DYNAL DSP. Use the MAXIO parameter on the SETPARAM statement to specify the maximum number of asynchronous I/O requests that can be processed simultaneously by the DYNAL DSP.

If you assign all allocation processing and all device names to JES3, identify all devices as either permanently resident or removable JES3-managed devices. JES3 then processes all requests, including dynamic allocation requests.

Note: All nonspecific, nonprivate requests requiring DASD space must be allocated from permanently-resident, or reserved devices. If insufficient space is available on such permanently-resident devices, allocation fails and no scratch volume is mounted.

To improve the performance of dynamic allocation, you can bypass integrity protection for selected data sets. JES3 then bypasses all MDS processing on the global processor for the selected data sets. To specify data sets that require integrity protection and those that do not, use the DYNALDSN initialization statement.

To reduce the number of spool I/O requests needed by dynamic allocation, suppress the messages that are sent to the JESMSGLG file for TSO users. To suppress these messages, code the JESMSG parameter on the STANDARDS initialization statement.