Example 4: Sizing local page data sets

Assume that the master scheduler address space and JES address space can each use about eight megabytes of private area storage. Next, determine the number of address spaces that will be used for subsystem programs such as VTAM® and the system component address spaces, and allow eight megabytes of private area storage for each. To determine the amount of space necessary for batch address spaces, multiply the maximum number of batch address spaces that will be allowed to be active at once by the average size of a private area (calculated by the installation or approximated at eight megabytes).

Note: If you are using storage-class memory (SCM), the size of your local page data sets can be reduced, assuming that SCM has demonstrated faster I/O response times.

To determine the amount of space necessary for TSO/E, multiply the maximum number of TSO/E address spaces allowed on the system at once by the average size of a private area (calculated by the installation or approximated at eight megabytes).

Next, determine the amount of space required for any large swappable applications which run concurrently. Use the allocated region size for the calculation.

Finally, estimate the space requirements for VIO data sets. Approximate this requirement by multiplying the expected number of VIO data sets used by the entire system by the average size of a VIO data set for the installation. After the system is fully loaded, you can use RMF™ reports to evaluate the estimates.

Note: If your local DASD storage is not large enough to contain your VIO data, VIO data will not be paged out to SCM.

For example purposes, assume that the total space necessary for local page data sets is:

megabytes for the master scheduler address space

megabytes for the PC/AUTH address space

megabytes for the system trace address space

megabytes for the global resource serialization address space

megabytes for the allocation address space

megabytes for the communications task address space

megabytes for the dumping services address space

megabytes for the system management facilities address space

megabytes for the VTAM address space

megabytes for the JES address space

megabytes for the JES3AUX address space if JES3 is used

megabytes for the batch address space (10 batches x 1 megabyte each)

megabytes for TSO/E address spaces (50 TSO/E users x 1 megabyte each)

102

megabytes for large swappable application

+ 40

megabytes for VIO data sets (200 data sets x 0.2 megabyte each)

290

megabytes total + 15 meg (approx. 5%) buffer = 305 megabytes

Therefore: From the tables, 549 cylinders on 3380 type devices are necessary. For the 3390, 434 cylinders are necessary.

Note: Even when the local page data sets will fit on one 3390, you should spread them across more than one device. See performance recommendation number 5. If large swappable jobs or large VIO users are started and there is insufficient allocation of space on local page data sets, a system wait X'03C' could result.

The installation should also consider the extent of the use of data-in-virtual when calculating paging data set requirements. Users of data-in virtual may use sizable amounts of virtual storage which may put additional requirements on paging data sets.

The calculations shown here will provide enough local page space for the system to run. However, if a program continually requests virtual storage until the available local page data set space is constrained, this will not be enough space to prevent an auxiliary storage shortage.

Auxiliary storage shortages can cause severe performance degradation, and if all local page data set space is exhausted the system might fail. To avoid this, you can do one or more of the following:

Use the IEFUSI user exit to set REGION limits for most address spaces and data spaces. If you have sufficient DASD, add enough extra local page data set space to accommodate one address space or data space of the size you allow in addition to the local page data set space the system usually requires, multiply the sum by 1.43, and allocate that amount of local page data set space. For more information about IEFUSI, see z/OS MVS Installation Exits.
Overallocate local page data set space by 300% if you have sufficient DASD. This is enough auxiliary storage to prevent the system from reaching the auxiliary storage shortage threshold when the maximum amount of storage is obtained in a single address space or data space.