Geographic mirroring

Geographic mirroring uses IBM® i cluster technology to provide a high availability solution where a consistent copy of data that is stored in an independent disk pool at the production system is maintained on a mirrored copy. Geographic mirroring maintains a consistent backup copy of an independent disk pool by using internal or external storage.

If an outage occurs on the production site, production is switched to the backup site, which contains the mirrored copy of data, typically located at another location. In synchronous delivery mode, data is mirrored before write operations complete on the production system and is typically used for applications that cannot suffer any data loss in the event of a failure. Data is still sent to the mirror copy before the write operation completes while in asynchronous delivery mode, however, control is returned to the application before the mirrored write actually makes it to the mirror copy.

A good reason for using the existing synchronous delivery mode, is if the application wants to make sure that all completed writes on the production side have made it to the mirror copy side.

Geographic mirroring provides logical page level mirroring between independent disk pools through the use of data port services. Data port services manages connections for multiple IP addresses, which provides redundancy and greater bandwidth in geographic mirroring environments.

Geographic mirroring allows for the production and mirrored copies to be separated geographically, which allows for protection in the event of a site-wide outage. When planning a geographic mirroring solution, the distance between production and mirrored independent disk pools might affect the application response time. Longer distances between the production and mirrored copies might cause longer response times. Before implementing a high availability solution that uses geographic mirroring, you must understand your distance requirements and the associated performance implications on your applications.

With asynchronous delivery mode, the application response times are not impacted as with synchronous delivery mode. Large amounts of latency can result in additional main storage and CPU resources needed for asynchronous delivery mode.

Geographic mirroring is a technology available with IBM PowerHA® SystemMirror® for i.

Geographic mirroring with asynchronous delivery mode is only available with PowerHA version 2.0 and above.