Sysplex data-sharing concepts and terminology

Buffer invalidation

When a database block or CI is updated by one IMS, IMS must make sure all copies of this block or CI in other IMS buffer pools are marked invalid.

To do this, the updating IMS issues an invalidate call. This call notifies the coupling facility that the content of a buffer has been updated and that all other copies of the buffer must be invalidated. The coupling facility then invalidates the buffer for each IMS that registered an interest in the buffer. This process is called buffer invalidation. Whenever IMS tries to access a buffer, it checks to be sure that the buffer is still valid. If the buffer is invalid, IMS rereads the data from DASD. Data integrity is thereby maintained.

Buffer invalidation works in all IMSplex database environments: DB/DC, DBCTL, and DB batch. In the sysplex environment, IMS supports buffer pools for VSAM, VSAM hiperspace, OSAM, and OSAM sequential buffering buffers.

Data-sharing groups

As with non-sysplex data sharing, the concept of a data-sharing group applies. The following figure shows a sample data-sharing group.

Figure 1. Sample data-sharing group

The data-sharing group has two operating systems. Each operating system contains an IMS control region, an FDBR, and an IRLM. The IMS systems in each operating system share the following resources:

• Databases
• A RECON dual copy data set
• One or more coupling facilities
• A single IRLM lock table structure (lock structure) in the coupling facility
• OSAM and VSAM buffer invalidate structures in the coupling facility (hereafter called an OSAM or VSAM structure)

Fast database recovery regions are not shared.

Communication in the data-sharing group is through the IRLMs connected to the coupling facility. Without a coupling facility, no valid sysplex data-sharing environment exists; IRLM does not grant global locks, even for non-sysplex data sharing. IRLM requires a coupling facility for sysplex data sharing. If a coupling facility is not available, IRLM allows only non-sysplex data sharing.

IMS connects to the structures in the coupling facility. Sysplex data sharing uses the OSAM and VSAM structures for buffer invalidation. This differs from non-sysplex data sharing, which does buffer invalidation using broadcasts (notifies) to each IMS. Sysplex data sharing uses the lock structure in the coupling facility to establish and control the data-sharing environment.

A data-sharing group is defined using the CFNAMES control statement in the DFSVSMxx member of the IMS PROCLIB data set. The lock, OSAM, and VSAM structures are named on the CFNAMES control statement.

Coupling facility

Figure 1 shows the coupling facility and the three structures in it that are used for sysplex data sharing.

Although the figure shows a single coupling facility, more than one are possible. Additional coupling facilities can be defined for backup. Or, to increase throughput, structures can be split across coupling facilities; the lock structure, for example, can be put on one coupling facility and OSAM and VSAM structures on another.

The OSAM and VSAM structures are used for buffer invalidation. For each block of shared data read by any IMS system connected to the coupling facility, an entry is made in the OSAM or VSAM structure. Each entry consists of a field for the buffer ID (known to z/OS as the resource name) and 32 slots. The slots are for IMS systems to register their interest in an entry's buffer. This makes it possible for as many as 32 IMS systems to share data. Note that the sysplex data-sharing limit of 32 IMS systems is the number of IMS systems that can connect to a structure; it is not the number of IMS systems that are running. It is also not the number of total connections that can be made to the coupling facility which is 255.

The lock structure is used to establish the data-sharing environment. For a data-sharing group, the first IMS system to connect to a lock structure determines the data-sharing environment for any IMS system that subsequently connects to the same lock structure. When identifying to the IRLM, IMS passes the names of the coupling facility structures specified on the CFNAMES control statement, plus the DBRC RECON initialization time stamp (RIT) from the RECON header. The identify operation fails for any IMS system not specifying the identical structure names and RIT as the first IMS.

If a structure fails (or you initiate a manual rebuilding of the structure), IMS tries to rebuild it. If IMS is unsuccessful or if the connection to a structure is lost, IMS quiesces data-sharing transactions and stops data sharing. After the rebuilding is complete, IMS tries to reconnect to the structure. If IMS reconnects successfully, IMS continues processing data-sharing transactions. If IMS fails to reconnect, data sharing remains stopped and IMS waits to reconnect until it is again notified that coupling facility resources are available.