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CTCDASD Communication Method

The CTCDASD method is a combination of the DASDONLY and CTCONLY methods using the DASD file or a coupling facility structure as a backup.

With the CTCDASD communication method, the MIMplex can consist of sysplexed and non-sysplexed images. This method is actually a combination of the CTCONLY and DASDONLY communication methods.

A DASD control file or a coupling facility structure control file is used as the base communication method during initialization and recovery situations. During initialization, all systems synchronize on the DASD or coupling facility structure control file. If all VCF communication requirements are met, CA MIM then automatically migrates from the DASD or coupling facility structure control file to the virtual control file (VCF) on the selected master system.

When VCF synchronization has completed, the VCF is passed on demand from the master system to client systems. The VCF is passed between systems using 3088 or ESCON CTC devices allocated to the CA MIM address space on each system. The CTC devices to be used by CA MIM are identified on CTCPATH statements.

Conceptually, a CTC device connects an I/O address on one processor to an I/O address on another processor. VCF data that is sent from one side is received on the other side, so every transmission consists of two operations, an outbound write operation from one side, and an inbound read operation on the other side. Data may be transmitted in either direction over a CTC path, but it travels only in one direction at any one moment.

When using the CTCDASD communication method, it is important to remember that the DASD or coupling facility structure control file is the base communication medium. Migration to the DASD or coupling facility structure control file occurs any time a re-synchronization of the MIMplex is required. If possible, CA MIM automatically migrates back to using the VCF when re-synchronization completes.

You must re-synchronize the MIMplex when any of the following events occur:

• Problems with the current master system start an automatic recovery migration.
• You issue a MIGRATE command to select a new master system.
• A system joins or rejoins the established MIMplex.
• We recommend that sites using the CTCDASD communication method be configured symmetrically so that multiple systems are eligible as master systems. This redundancy provides the best recovery options.