Select guaranteed space by setting the storage class Guaranteed Space attribute to YES. SMS uses the supplied volume count when allocating the data set.

With the IBM® ESS, the Guaranteed Space attribute of a storage class with specific volsers is no longer required for data sets other than those that need to be separated, such as the DB2® online logs and BSDS, or those that must reside on specific volumes because of their naming convention, such as the VSAM RLS sharing control data sets. The ESS storage controllers use the RAID architecture that enables multiple logical volumes to be mapped on a single physical RAID group. If required, you can still separate data sets on a physical controller boundary for availability beyond what is inherently built into the RAID architecture.

The ESS is also capable of parallel access volumes (PAV) and multiple allegiance. These ESS capabilities, along with its bandwidth and caching algorithms, make it unnecessary to separate data sets from each other for the purpose of performance. Traditionally, IBM storage subsystems allow only one channel program to be active on a disk volume at a time. This means that after the subsystem accepts an I/O request for a particular unit address, this unit address appears "busy" to subsequent I/O requests. This ensures that additional requesting channel programs cannot alter data that is already being accessed. By contrast, the ESS is capable of multiple allegiance, or concurrent execution of multiple requests from multiple hosts. That is, the ESS can queue and concurrently execute multiple requests for the same unit address from multiple hosts, provided that no extent conflict occurs.

In addition, the ESS enables PAV or multiple concurrent accesses to a single volume from a single host. To access a volume concurrently, you must associate multiple device numbers with a single volume. The ESS provides this capability by allowing you to define a PAV-base address and one or more PAV-alias addresses. It allows up to 255 aliases per logical volume. Therefore, you no longer have to separate data sets from each other for performance reasons.

If you specify NO for Guaranteed Space, then SMS chooses the volumes for allocation, ignoring any VOL=SER statements specified on JCL. Primary space on the first volume is preallocated. NO is the default.