Understanding ACS routines

ACS routines can be used to determine the SMS classes and storage groups for data sets and objects in an SMS complex. For storage administrators, ACS routines automate and centralize the process of determining SMS classes and storage groups. ACS routines also help convert data sets to an SMS environment.

An object is assigned to a storage group when it is stored and remains in that storage group throughout its lifetime. The initial storage class and management class might be determined by defaults defined by an ACS routine or by explicit request. Storage class and management class assignments might be changed by the OSREQ CHANGE function or by automatic class transition. The OSREQ CHANGE request causes invocation of ACS routines that might override the requested assignments. During the automatic class transition, ACS routines are invoked to determine the new storage class and management class assignments.

When a data set is allocated, it is assigned an initial management class, storage class, and storage group by defaults defined by an ACS routine or by explicit request. These assignments can be changed through the DFSMShsm class transition processing that occurs during the automatic space management functions. During the automatic class transition, ACS routines are invoked with the ACS environment of SPMGCLTR to determine the new management class, storage class, and storage group assignments.

Writing ACS routines lists the rules for programming in the ACS language.

Tip: You can use the DFSMS NaviQuest tool to help you design and test your ACS routines. First, you can create test cases to perform extensive testing against test data representing actual data sets. Then you can test ACS routines in batch, freeing the workstation for other work. See Using NaviQuestfor further information.

Through ISMF, you can create and maintain as many as four ACS routines in an SCDS, one for each type of SMS class and one for storage groups. After you have activated an SMS configuration, SMS executes ACS routines for the following operations:

JCL DD statements (DISP=NEW, DISP=MOD)
Dynamic allocation requests (DISP=NEW, DISP=MOD for a nonexistent data set)
DFSMSdss COPY, RESTORE, and CONVERTV commands
DFSMShsm RECALL, RECOVER, and class transitions
Access method services ALLOCATE, DEFINE, and IMPORT commands
OAM processing for STORE, CHANGE, and class transition
Local data set creation by remote application through Distributed FileManager/MVS
MVS data sets or z/OS® UNIX System Service (z/OS UNIX) files created by remote application through the z/OS Network File System server

As a storage administrator, you write ACS routines using the ACS programming language, a high-level programming language. The language follows a logical, procedural flow of implementation that consists mainly of filtering criteria, IF/THEN statements, and SELECT/WHEN statements. Using these relational statements, ACS routines determine SMS classes and storage groups according to allocation parameters, data set sizes, object or data set names, and other variables.

All allocations directed to units that are neither tape nor DASD should be excluded from SMS management. Do this by testing for UNIT in the storage class routine and ensuring that the storage class is set to NULL in these cases.

Ensuring that no storage class is assigned for such allocations avoids potential errors with allocations that require specific types of units. For example, assigning a storage class to a VTAM® channel-to-channel (CTC) adaptor allocation results in sense errors when VTAM attempts to use the CTC.

Requirements: For system-managed data sets, the storage group is required because there is no way to explicitly specify storage groups. The other routines are optional for system-managed data sets. For objects, the storage group, storage class and management class ACS routines are required. For tape, the storage group, storage class and data class ACS routines are required.