Advanced Topics › CTCDASD as the Initial Communication Method

CTCDASD as the Initial Communication Method

The CTCDASD initial communication method is a combination of physical control files and VCF methods using the DASD file or a coupling facility structure as a backup.

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
• An inbound read operation on the other side

Data can 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 resynchronization of the MIMplex is required. If possible, CA MIM automatically migrates back to using the VCF when resynchronization completes.

Note: Although you may have specified CTCDASD as your initial communication method, CA MIM will automatically join an XCF group. CA MIM will build XCF communication paths for all other active members of the CA MIM XCF group. You can instruct each client system to use an XCF path to the master system (if available) by setting your VCFPREFERENCE.