Paging operations and algorithms

To page efficiently and expediently, ASM divides z/OS® system pages into classes, namely PLPA, common and local. Contention is reduced when these classes of pages are placed on different physical devices. Multiple local page data sets are recommended. Although the system requires only one local page data set, performance can be improved when local page data sets are distributed across multiple devices, even if one device is large enough to hold the entire amount of required page space.

The PLPA and common page data sets are optional if storage-class memory (SCM) is available (specify *NONE* to use SCM), but there can be only one of each. Spillage back and forth between the PLPA and common page data sets is permissible, but in the interest of performance, only spilling from PLPA to common should be permitted.

The general intent of the ASM algorithms for page data set selection construction is to:

Use all available local page data sets: When ASM writes a group of data, it selects a local page data set in a circular order within each type of device, considering the availability of free space and device response time.
When ASM selects a data set, the paging data sets that reside on Parallel Access Volume (PAV) devices are examined first because of reliability and performance characteristics. Because preference is given to PAV devices, it is normal to have a higher usage of PAV data sets as compared to non-PAV data sets.
Write group requests to contiguous slots: ASM selects contiguous space in local page data sets on moveable-head devices to receive group write requests. For certain types of requests, ASM's slot allocation algorithm tries to select sequential (contiguous) slots within a cylinder. The reason for doing this is to shorten the I/O access time needed to read or write a group of requests. For other types of requests (such as an individual write request), or if there are no sequential slots, ASM selects any available slots.
Limit the apparent size of local page data sets to reduce seek time: If possible, ASM concentrates group requests and individual requests that are within a subset of the space allocated to a local page data set.