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.
If a failover operation is initiated, the application must recover and apply any updates that were not committed to the secondary volume. If I/O operations on the primary volume are paused for a small length of time, the secondary volume can become an exact match of the primary volume. This function is comparable to a continuous backup process in which the last few updates are always missing. When you use Global Mirror for disaster recovery, you must consider how you want to handle these missing updates.
To use the Global Mirror function, all components in the network must be capable of sustaining the workload that is generated by application hosts and the Global Mirror background copy process. If all of the components in the network cannot sustain the workload, the Global Mirror relationships are automatically stopped to protect your application hosts from increased response times.
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.