Metro Mirror and Global Mirror

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.

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.

If you are using Global Mirror with cycling mode set to Multiple, the copying process is similar to Metro Mirror and standard Global Mirror. Change volumes must be configured for both the primary and secondary volumes in each relationship. A copy is taken of the primary volume in the relationship using the change volume that is specified when the Global Mirror relationship with change volumes is created. The background copy process reads data from the stable and consistent change volume, copying the data to the secondary volume in the relationship. Copy-on-write technology is used to maintain the consistent image of the primary volume for the background copy process to read. The changes that took place while the background copy process was active are also tracked. The change volume for the secondary volume can also be used to maintain a consistent image of the secondary volume while the background copy process is active.