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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.

Characteristics of geographic mirroring
  • All data maintained in the independent disk pool will be replicated to a second copy of the data on a second system.
  • Replication is a function of the IBM i OS so any type of storage can be used.
  • The application can be switched to the backup system and operate on the independent disk pool copy.
  • Two copies of the data eliminating single point of failure.
  • When using synchronous geographic mirroring, both copies of the IASP are guaranteed to be identical. Synchronous geographic mirroring over a distance may impact application performance due to communication latency.
  • Second copy of data can be geographically dispersed if using asynchronous geographic mirroring. In the case of an unplanned outage on the source system, a few seconds of data loss is possible.
  • Data transmission over 1 to 4 TCP/IP communication lines for throughput and redundancy.
  • It is also recommended that a separate line be used for the clustering heartbeat since sharing the heartbeat with data port can cause contention and time outs.
  • Offline saves and queries to backup copy of the data while backup dataset is detached.
  • Data resiliency not maintained while backup dataset is detached. Data resiliency is resumed after partial or full resynchronization has completed.
  • Can be used in conjunction with the IBM i switch LUN technology.
  • System performance overhead is associated with running geographic mirroring.
  • It is strongly recommended that you configure separate main storage pools or user jobs that access independent disk pools in order to prevent those jobs from contending with other jobs on the system and using more main storage than desired. More specifically, independent disk pool jobs should not use the machine pool or base pool. If independent disk pool jobs use the same memory as jobs that are not accessing the independent disk pools, independent disk pool jobs can monopolize the memory pool, lock out other jobs, and in extreme situations deadlock the system. Exposure for this situation is greater when using geographic mirroring.
  • Journaled objects in the independent disk pool will guarantee data update to target system.
  • Simple monitoring of mirror process.
  • Cost associated with a second set of disk.
  • Replication is at a memory page level managed by IBM i.
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