How Metro/Global Mirror works

To understand how Metro/Global Mirror works, it is important to understand how the volumes are configured at each site.

Figure 1 shows a Metro/Global Mirror environment that includes the A, B, C, and D volume configuration at the three sites.

There are three volume configurations in a Metro/Global Mirror environment:

Volumes at the local (or primary) site are Metro Mirror primary (or source) volumes and are referred to as the A volumes.
Volumes at the intermediate site are the Metro Mirror secondary (or target) volumes and Global Mirror primary volumes at the same time. These volumes are referred to as the B volumes. (If additional volumes are used at the intermediate site as FlashCopy target volumes, they are referred to as E volumes.)
One set of volumes at the remote site are the Global Mirror secondary volumes and FlashCopy source volumes at the same time. These volumes are referred to as the C volumes. The FlashCopy target volumes are referred to as the D volumes.
Figure 1. Metro/Global Mirror volume configuration

The Global Mirror function allows you to combine a Global Copy volume pair with FlashCopy such that the Global Copy secondary also serves as the FlashCopy source in a Global Mirror session. FlashCopy plays an integral part of this configuration because the set of volumes that are FlashCopy target volumes contain consistent data. This set of volumes is called a consistency group.

A Metro/Global Mirror environment is configured using a minimum of three storage units, one each at the local, intermediate, and remote sites. Data from the A volumes at the local site is synchronously replicated to B volumes at the intermediate site using Metro Mirror. Data from the B volumes at the intermediate site is asynchronously replicated to the C volumes at the remote site using Global Copy. FlashCopy relationships that are created with the C volumes at the remote site become the FlashCopy source volumes and the D volumes at the remote site become the FlashCopy target volumes.

Note: For ease of understanding, we describe the Metro/Global Mirror configuration in terms of the local, intermediate, and remote sites. You can manage the volumes that are associated with the storage units at these sites from one central location.

For Metro/Global Mirror, one storage unit at the intermediate site is designated as the master. Optional additional storage units at the intermediate site, if configured, are designated as subordinates. The master sends commands to its subordinate storage units over Fibre Channel protocol (FCP) links and coordinates the consistency group formation process over all the subordinate storage units. The FCP links also enable the Global Mirror master storage unit to communicate the FlashCopy commands to the remote site. All status is then relayed back to the master storage unit. Figure 2 provides a high-level view of the three-site Metro/Global Mirror configuration.

Figure 2. Metro/Global Mirror configuration with master and subordinate storage units

In a Metro/Global Mirror configuration, if you lose access to the storage unit at the local site, the Global Copy relationship is intact and Global Mirror is still operational. Through the use of FlashCopy operations and consistency groups, consistent data is maintained at the remote site. However, you are left without the local site until all recovery operations have been performed to recover the local site.

A three-site Metro/Global Mirror configuration provides the following recovery options at the alternate sites if a failure occurs:

If an outage occurs at the local site, recovery operations can begin at the intermediate site. Global Mirror continues to mirror updates between the intermediate and remote sites, maintaining the recovery capability at the remote site.
If an outage occurs at the local site, recovery operations can begin at the remote site and preparations can be made to resynchronize the local site when it recovers from its disaster. Once in recovery mode at the remote site, another Global Mirror session can be set up and put into operation using the former intermediate as its new remote site. This new Global Mirror session provides additional disaster recovery while operating at the remote site.
If an outage occurs at the intermediate site, data at the local storage unit is not affected. Applications continue to run normally.
If an outage occurs at the remote site, data at the local and intermediate sites is not affected. Applications continue to run normally. The intermediate storage unit maintains a consistent up-to-date copy.
If both the local and intermediate sites are lost, the scenario is like a two-site scenario when access to the local storage unit is lost. Recovery must be achieved using the last consistent point-in-time copy at the remote site.