When you define a pool storage group, you specify the volume serial numbers of the DASD volumes you are including in the storage group rather than their physical addresses. Each DASD volume in a storage group must contain a VSAM volume data set (VVDS) and an indexed volume table of contents (VTOC). The VVDS is automatically created after allocation of the first system-managed data set on a volume, if the VVDS is not already defined.

Two storage groups cannot share a DASD volume. You must define an entire volume to a single pool storage group. Also, a data set can only reside in one pool storage group. A data set can span volumes within a single pool storage group, but it cannot span volumes belonging to several pool storage groups. For VSAM data sets, the entire sphere (base cluster and all alternate indexes) must be in the same storage group.

Although you can separate devices according to geometry, you do not need to separate them according to capacity. For example, you can combine all models of the 3390 into a single storage group. The only effect the different capacities has is on volume thresholds. See z/OS DFSMS Implementing System-Managed Storage for information on selecting appropriate threshold levels.

Devices of the same geometry can have different performance characteristics. These devices coexist in the same storage group, and enhanced volume selection for SMS manages data set placement accordingly. Even devices with vastly different performance characteristics can reside in the same storage group.

For striped data sets through sequential access, the effective data rate is limited by the slowest stripe. If a storage group contains volumes with widely different data delivery capabilities, such as 3990 Model 6 and ESS, the effective data rate for striped data sets through sequential accesses is gated by the stripes on 3990 Model 6.