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 Metro Mirror function supports copy operations between volumes that are separated by distances up to 300 km. For disaster recovery purposes, Metro Mirror provides the simplest way to maintain an identical copy on both the primary and secondary volumes. However, like with all synchronous copies over remote distances, there can be a performance impact to host applications. This performance impact is related to the distance between primary and secondary volumes and depending on application requirements, its use might be limited based on the distance between sites.