Remote-copy function

This remote-copy type creates a synchronous copy of data from a master volume to an auxiliary volume. An auxiliary volume can either be on the same system or on another system. Metro Mirror can be used between volumes that are separated by distances up to 300 km.

With synchronous copies, host applications write to the master volume but do not receive confirmation that the write operation completes until the data is written to the secondary volume. With synchronous copies, both the volumes have identical data when the copy operation completes. After the initial copy operation completes, the Metro Mirror copy type maintains a fully synchronized copy of the source data at the target site.

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, performance on host applications can be impacted. 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 remote-copy type 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 auxiliary volume. If I/O operations on the primary volume are paused for a small length of time, the auxiliary 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 sustain 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 auxiliary volume as soon as possible after they are applied to the primary volume. The auxiliary 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 sites.

If you are using Global Mirror with change volume, the copying process is similar to Metro Mirror and standard Global Mirror. Change volumes must be configured for both the primary and auxiliary volumes in each relationship. A copy is taken of the primary volume in the relationship that uses 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 auxiliary 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 auxiliary volume can also be used to maintain a consistent image of the auxiliary volume while the background copy process is active. Global Mirror with change volumes help eliminate the missing updates that can occur when you use Global Mirror without change volumes.

Change volumes can be used in a number of cases. Global Mirror relationships with cycling mode set to Multiple must always be configured with change volumes. Global Mirror relationship with cycling mode set to None can optionally be configured with change volumes, which can be used to maintain a consistent secondary image.