Geographic mirroring

Geographic mirroring is a function of the IBM® i operating system. All the data that is placed in the production copy of the IASP is mirrored to a second IASP on a second, perhaps remote system. The replication is done within the operating system, so this solution can be used with any type of storage. There is both a synchronous and asynchronous version of geographic mirroring. Synchronous geographic mirroring guarantees that the two copies of the data are identical, but has a distance limitation, since the IO transaction will not complete on the source system until the IO has also reached the target system. Asynchronous geographic mirroring has no distance limitation, but if the source side fails unexpectedly, there may be a few seconds of data loss.

The benefits of this solution are essentially the same as the switched LUN solution with the added advantage of providing disaster recovery to a second copy at increased distance. The biggest benefit continues to be operational simplicity. The switching operations are essentially the same as that of the switched LUN solution, except that you switch to the mirror copy of the IASP, making this a straightforward HA solution to deploy and operate. As in the switched LUN solution, objects not in the IASP must be handled by some other mechanism such as administrative domain, and the IASP cannot be brought online to an earlier system. Geographic mirroring also provides real-time replication support for hosted integrated environments such as Microsoft Windows and Linux. This is not generally possible through journal-based logical replication.

Since geographic mirroring replication is within the IBM i operating system, a potential limitation of a geographic mirroring solution is performance impacts in certain workload environments. For synchronous geographic mirroring, when running input/output (I/O) intensive batch jobs, some performance degradation on the primary system is possible. Also, be aware of the increased central processing unit (CPU) overhead that is required to support geographic mirroring.

The backup copy of the independent disk pool cannot be accessed while the data synchronization is in process. For example, if you want to back up to tape from the geographically mirrored copy, you must quiesce operations on the source system and detach the mirrored copy. Then you must vary on the detached copy of the independent disk pool on the backup system, perform the backup procedure, and then reattach the independent disk pool to the original production host. Synchronization of the data that was changed while the independent disk pool was detached will then be performed. Your HA solution is running exposed, meaning there is no up-to-date second data set, while doing the backups and when synchronization is occurring. Geographic mirroring utilizes source and target side tracking to minimize this exposure.