Allocating control data sets

Before you can activate an SMS configuration, you need to allocate control data sets used by SMS and define their contents. Control data sets are virtual storage access method (VSAM) linear data sets that contain:

Base configuration information
SMS class, aggregate group, optical library, tape library, optical drive, and storage group definitions
ACS routines.

You can allocate control data sets with access method services or TSO/E commands. You can define and alter the contents of control data sets using ISMF. SMS uses three types of control data sets: a source control data set (SCDS), an active control data set (ACDS), and a communications data set (COMMDS).

Restriction: Do not name any of your SMS control data sets the single word ‘ACTIVE’. SMS uses the single word ‘ACTIVE’ as a reserved word indicating the active configuration residing in the SMS address space. Naming an SMS control data set ‘ACTIVE’ results in errors.

Source control data set (SCDS):

An SCDS contains an SMS configuration, which defines a storage management policy. You can define any number of SMS configurations each of which has its own SCDS. Then, you select one SMS configuration to be the installation storage management policy and make an active working copy of it in an ACDS.

Active control data set (ACDS):

When you activate an SCDS, its contents are copied to an ACDS. The current ACDS contains a copy of the most recently activated configuration. All systems in an SMS complex use this configuration to manage storage. You can define any number of SCDSs, but only one can be put in the ACDS. z/OS MVS Initialization and Tuning Guide explains how to specify the ACDS. You can define more than one IGDSMSxx member, each specifying a different ACDS, but you can use only one ACDS at a time.

Restriction: You cannot define or alter an ACDS. This also means that you cannot use an ACDS as an SCDS if the SCDS is lost.

You can modify the SCDS from which your current storage management policy was activated without disrupting operations, because SMS manages storage with a copy of the SMS configuration (an ACDS) rather than with the original (an SCDS). While SMS manages storage using an ACDS, you can:

Create a backup copy of the SCDS
Build a new SCDS
Update the SCDS from which the ACDS was activated
Modify any SCDS

Figure 1 shows the relationship among SCDSs and ACDSs in an installation.

Figure 1. Relationships among SCDSs and ACDSs in an installation

Communications data set (COMMDS):

The COMMDS serves as the primary means of SMS communication among systems in the SMS complex. The active systems in an SMS complex access the COMMDS for current SMS complex information.

The COMMDS contains the name of the ACDS containing the currently active storage management policy, the current utilization statistics for each system-managed volume, and other system information. You can define any number of COMMDSs, but only one can be active in an SMS complex.