IMS system data sets for online change

Planning for the definition of IMS data sets requires you to predict the direct access storage to be allocated based on your anticipated application program work load. Some of the data sets you estimate are affected by the design decisions made for system definition. Most of these data sets are automatically generated as DD statements within the members of IMS.PROCLIB.

Note: Certain target and distribution library data sets must be either a PDS or a PDSE. The ADFSJLIB, SDFSJLIB, and ADFSLOAD data sets must be PDSE data sets. SDFSRESL data set must be a PDS. All other target (SDFSxxxx) and distribution (ADFSxxxx) libraries can be either PDS or PDSE.

Restriction: IMS supports partitioned data sets extended (PDSEs) for only these libraries: MODBLKS, PGMLIB, SMPLTS, and SDFSJLIB.

The External Subsystem Attach Facility (ESAF) supports PDSE load library data sets.

IMS provides the following methods for adding, deleting, and replacing certain resources online without shutting down your IMS system:

Dynamic resource definition (DRD)
Online change (OLC)
When the IMS management of ACBs is enabled, the IMPORT DEFN command for application control blocks (ACBs)

An online change performs changes to a local IMS (called local online change) or to IMS systems in an IMSplex (called global online change). For an overview of performing online changes in these environments, see The online change function.

When the IMS management of ACBs is enabled, the online change function is not supported for ACBs and you do not need to allocate ACBLIB data sets. Instead, the ACBs, which represent the databases (DBDs) and program views (PSBs) in the online system, are managed by IMS in the IMS catalog. You activate most ACB changes in an online system by issuing the IMPORT DEFN SOURCE(CATALOG) command. For more information about enabling the IMS management of ACBs, see IMS management of ACBs.

Adding, deleting, or changing IMS resources involves changes to the control blocks set up for these resources. If you use online change, making additions, deletions, or changes requires a MODBLKS system definition. Within a MODBLKS system definition, you specify changes to keyword parameters on the DATABASE, APPLCTN, TRANSACT, and RTCODE macro statements. When designing a DBCTL or DCCTL environment, use the information in this topic as it applies to your system. For a DBCTL environment, no MFS facility exists, and the TRANSACT and RTCODE macros do not apply. A DCCTL environment has no database facilities; therefore, the DATABASE keyword does not apply. A MODBLKS system definition generates the control block members for resources that are to be added or changed online. These control blocks, stored in the IMS.MODBLKS data set, are used by the IMS control region and the MSC Verification utility when an online change to your IMS system is requested.

Before you can use online change, you must create three copies of each of the following libraries:

IMS.MODBLKS—the library that contains the control blocks to support online change of databases, programs, transactions, and MFS formats
IMS.ACBLIB—the library that contains database and program descriptors
IMS.FORMAT—the library that contains your MFS maps produced by the MFS Language and Service utilities

These libraries are for the exclusive use of IMS offline functions and are called the staging libraries. Two copies are made of each library, producing data sets with a data set name suffixed with an A and a B, for example, IMS.FORMATA and IMS.FORMATB. These two copies of each library are used by the IMS online system.

Note: If you have multiple copies of the staging IMS.ACBLIB in an IMSplex, each copy of the IMS.ACBLIB must be identical.

When IMS IVP processing completes, the staging libraries and the IMS A libraries are identical, and the A libraries are referred to as the active libraries. IMS draws its execution information from the A libraries. The B libraries, which are not used at this time, are referred to as the inactive libraries.

Figure 1 illustrates how libraries are used when you change your system online:

You apply changes to the staging libraries.
The staging libraries are then copied to the inactive (B) libraries using the Online Change utility.
Operator commands are issued to cause the B libraries to become the active libraries; the old active (A) libraries become the inactive libraries.

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The process above is repeated as necessary. When you choose to add, replace, or delete any IMS resources, you apply your changes to the offline staging libraries by running either:

A MODBLKS system definition, if you have added, changed, or deleted application programs, full-function databases, DEDBs, or routing codes
An ACBGEN, if you have added, changed, or deleted any databases or programs
The MFS Language and Service utilities, if you have added, changed, or deleted any MFS format definitions

You can apply changes to IMS.FORMAT, IMS.ACBLIB, or IMS.MODBLKS independently or in combination. IMS.MODBLKS is changed by the MODBLKS system definition.

After the sequence of commands (/MODIFY for local online change or INITIATE OLC for global online change) has been issued to cause the previously inactive libraries to become the active libraries, your previously active libraries now become the inactive libraries. They are not destroyed until they are overwritten by the next online change sequence. You can return to the inactive libraries if back up and recovery are necessary, or if an incorrect definition occurs during your online change.

IMS monitors which set of libraries is currently active. If local online change is enabled, this information is kept in a status data set, IMS.MODSTAT. If global online change is enabled, this information is kept in the IMSPLEX.OLCSTAT data set. All IMS systems in an IMSplex that share the IMSPLEX.OLCSTAT data set must specify the same value on the ACBSHR= parameter, which is specified on both the DFSCGxxx and DFSDFxxx PROCLIB data set members.

After an online change successfully completes, it persists across all types of IMS restarts. Additionally, the new resources can be easily maintained by running an SMP/E JCLIN against the Stage 1 output stream produced by your MODBLKS system definition to record the contents of the new system definition in your SMP/E control data set. This ensures that any maintenance applied to your IMS system is applied to the currently active IMS system. Do not manage the online change data sets with a migration/recall system that might recall the data set to a volume other than the one to which it was originally allocated. If you do so, IMS might be unable to warm start or emergency start the system.

JES considerations

If you use JES3, include all IMS data sets and databases in the RESDSN statement.