This chapter explains the basic characteristics of the DS8000 Metro Mirror function and includes the following topics:

Metro Mirror is a synchronous replication solution between two DS8000s where write operations are completed on both the local and remote volumes before the I/O is considered to be complete. Metro Mirror is used in environments that require no data loss in the event of a storage system failure.

The volume written to by the host server is referred to as the primary or source volume. The remote volume to which the data is mirrored is called the secondary or target volume. The terms primary/secondary and source/target are used interchangeably.

Figure 13-1 illustrates the sequence of operations for a write operation to a Metro Mirror primary volume.

When the application performs a write update operation to a Metro Mirror primary volume, the following actions occur:

During normal operation with Metro Mirror, all data written to the primary volume is mirrored to the secondary volume, keeping the contents of the two volumes identical.

Because data is synchronously transferred to the secondary storage system before considering the write to be complete, the distance between primary and secondary storage systems affects the application response time for writes. The maximum supported distance for Metro Mirror is 300 km.

Read operations access only the primary storage system and are not affected by the mirroring.

Metro Mirror is generally used for disaster recovery purposes to avoid data loss in the event of a failure. Because Metro Mirror transmits data to the secondary synchronously with the application I/O, the data on the secondary is the same as on the primary. In the event of a failure at the primary production site, applications can be continued using the data from the secondary storage system. When combined with management software, such as IBM Copy Services Manager or IBM Geographically Dispersed Parallel Sysplex (IBM GDPS), Metro Mirror provides a high availability function for the transparent failover of host I/O operations.

Metro Mirror can also be used for migrating data from one storage system to another.