Metro Mirror and Global Mirror are two types of remote-copy operations. You can use these functions to set up a relationship between two volumes, where updates made to one volume are mirrored on the other volume. For Metro Mirror and Global Mirror operations, the volumes can be on two different systems (intersystem). Metro Mirror can also support volumes that reside on the same system (intrasystem). To use Metro Mirror and Global Mirror functions, the remote-copy license must be installed on each enclosure that you plan to use these functions.
Although data is only written to a single volume, the system maintains two copies of the data. If the copies are separated by a significant distance, the Metro Mirror and Global Mirror copies can be used as a backup for disaster recovery. A prerequisite for Metro Mirror and Global Mirror operations between systems over Fibre Channel connections is that the SAN fabric to which they are attached provides adequate bandwidth between the systems. SAN fabrics are not required for IP-only connections.
For both Metro Mirror and Global Mirror copy types, one volume is designated as the primary and the other volume is designated as the secondary. Host applications write data to the primary volume, and updates to the primary volume are copied to the secondary volume. Normally, host applications do not run I/O operations to the secondary volume.
Metro Mirror is a type of remote copy that creates a synchronous copy of data from a primary volume to a secondary volume. A secondary volume can either be on the same system or on another system. With synchronous copies, host applications write to the primary volume but do not receive confirmation that the write operation has completed until the data is written to the secondary volume. This ensures that both the volumes have identical data when the copy operation completes. After the initial copy operation completes, the Metro Mirror function maintains a fully synchronized copy of the source data at the target site at all times.
The Global Mirror function provides an asynchronous copy process. When a host writes to the primary volume, confirmation of I/O completion is received before the write operation completes for the copy on the secondary volume.
When Global Mirror operates without cycling, write operations are applied to the secondary volume as soon as possible after they are applied to the primary volume. The secondary volume is generally less than 1 second behind the primary volume, which minimizes the amount of data that must be recovered if a failover occurs. However, a high-bandwidth link must be provisioned between the two sites.
As part of system administration, you can change the copy type of relationships and consistency groups between Metro Mirror and Global Mirror. To do so, you can use the management GUI or CLI commands. For more information, see Changing the copy type of remote-copy relationships and consistency groups.