Characteristics of primary and secondary structures

z/OS® commands let you stop and start duplexing, and let you choose which of the duplexed structures is the primary and which is the secondary.

The primary structure is the one from which changed data is cast out to disk. Db2 also uses the primary structure to read data, keep track of page-registration, and perform cross-invalidation of pages in the buffer pools of individual members. When changed data is written to the primary structure, it is also written to the secondary structure. Db2 uses the secondary structure as a backup. After data is cast out to disk from the primary structure, the data is deleted from the secondary structure.

From a z/OS perspective, duplexing is really an extended rebuild, so z/OS documentation and commands sometimes call the primary structure the old structure, and refer to the secondary structure as the new structure.