Page data set sizes

The size of a page data set can affect system performance. The maximum number of slots that a page data set can be is 16,777,215. However, the amount of available free space on the volume that the page data set is allocated to limits the size that can be allocated. A 3390 device with 65,520 cylinders contains 11,793,600 slots.

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

Note the following recommendations:

PLPA data set. The total combined size of the PLPA page data set and common page data set cannot exceed the size of the PLPA (and extended PLPA) plus the amount that is specified for the CSA and the size of the MLPA. In defining the size of these data sets, a reasonable starting value might be four megabytes for PLPA and 20 megabytes for common, as spilling occurs if the PLPA data set becomes full.
RMF™ reports can be used to determine the size requirements for these data sets. During system initialization, check the size of the page data sets in the page data set activity report. If the values of the used slots are very close to the values of the allocated slots, the size should be enlarged. For more information, see the activity report of the page data set in z/OS RMF Report Analysis.
Common page data set. The common page data set should be large enough to contain all of the common area pages plus room for any expected PLPA spill. Although it is possible for the common page data set to spill to the PLPA page data set, this situation should not be allowed to occur because it might heavily affect performance. As noted for the PLPA data set, a reasonable starting size for the common page data set might be twenty megabytes.
Local page data sets. The local page data sets must be large enough to hold the private area and VIO pages that cannot be contained in real storage. To ensure an even distribution of paging requests across as many data sets as possible, all local paging data sets should be approximately the same size.
Note: When all local paging data sets are not the same size, the smaller data sets might reach a full condition sooner than the large data sets. This reduces the number of data sets that are available to ASM for subsequent paging requests. Depending on the paging configuration, this situation could degrade paging performance because the I/O workload and contention for the remaining available data sets might increase.
To minimize the path length in the ASM slot selection code (bitmap search), plan for local page data sets to not exceed 30% of their capacity under normal system workloads.