System-managed list structure duplexing

System-managed coupling facility duplexing provides a general-purpose, hardware-assisted mechanism for duplexing coupling facility structure data.

It is a robust mechanism that provides recovery from failures such as loss of a single structure or coupling facility, or from loss of connectivity, by a rapid switch to the other structure in a duplex pair.

Unlike system-managed rebuild, which can rebuild only from one structure to another, and is not applicable when the original structure has been lost or damaged, system-managed duplexing creates and maintains a duplexed copy of a structure in advance of any failure. It is largely transparent to the users of the structure, and is available for both planned and unplanned outages of a coupling facility or structure. For example, until CICS® TS 2.2, coupling facility data tables and temporary data sharing structures have contained mostly scratch-pad information that is not critical to recover. With duplexing, you can use the coupling facilities to store more critical information.

Transactions that are accessing a duplexed structure might experience delays, which could result in the DTIMOUT threshold being reached, when:
  • A structure is quiesced by MVS while duplexing is being established.
  • A structure is quiesced as a result of an operator command to stop or start duplexing.
  • A structure is switched to the secondary structure or back to the primary structure.

A newly-started data sharing server is not allowed to connect to a structure during any phase of a duplexing rebuild until the duplex established phase is reached.

System-managed duplexing is built on the system-managed rebuild function. However, whereas system-managed rebuild requires OS/390® with APAR OW39892, system-managed duplexing requires z/OS® Version 1 Release 2 with an enabling APAR. You also need to specify the DUPLEX option, in the CFRM policy information for the structure, as either DUPLEX(ENABLED) or DUPLEX(ALLOWED). If you specify DUPLEX(ENABLED), MVS will initiate and attempt to maintain a user-managed duplexing operation for the structure. If you specify DUPLEX(ALLOWED), an operator can start duplexing, using the SETXCF START,REBUILD,DUPLEX command. You also need to ensure that coupling facilities that are taking part in duplexing operations communicate with one another through a peer link.

For more information about system-managed duplexing, see z/OS MVS Programming: Sysplex Services Guide and z/OS MVS Setting Up a Sysplex.