Disk mirroring

VSAM database support provides mirrored disks and the ability to balance disk queues to provide a high level of data availability for z/TPF applications.

With disk mirroring, a disk failure remains transparent to the application as long as the mirror disk can manage the request. For example, if a volume of an index or data space is taken offline either manually or by the z/TPF system, VSAM database support switches active read operations off the failed volume to the alternate volume on the mirror data cluster. In the same way, any read operations that occur after the disk failure are switched to the alternate disk. When the disk volume or cluster is brought online again, VSAM database support incorporates the disk or cluster into the configuration again and uses it immediately.

Disks can be added or removed from the configuration at any time by using the ZDSMG DM or ZDSMG MT command with the ALL parameter specified for an entire cluster or the VOLUME parameter specified for an individual disk. Note that the index and data portions must be handled separately for a KSDS; therefore, you must enter the ZDSMG command twice, each time with a separate data definition (DD) name reference.

Each VSAM cluster must be identical to the other for disk mirroring to work, except for the volume serial numbers (VOLSERs). A z/OS® database administrator must run two jobs to create two VSAM clusters on different sets of disk volumes to set up a mirrored VSAM group. The z/OS DEFINE command in the VSAM CREATE function is identical for both clusters except for the VOLSERs, which must change. This ensures that VSAM arranges the files in exactly the same way on both sets of disks. Each disk then has a unique z/OS data set name.