The Metro/Global Mirror function combines the capabilities of Metro Mirror and Global Mirror functions for greater protection against planned and unplanned outages.
Metro/Global Mirror is a three-site, high availability disaster recovery solution, which uses synchronous replication to mirror data between a local site and an intermediate site, and asynchronous replication to mirror data from an intermediate site to a remote site. The DS8000 series supports the Metro/Global Mirror function on open systems and System z® or S/390® hosts. You can set up and manage your Metro/Global Mirror configurations using DS CLI and TSO commands.
In a Metro/Global Mirror configuration, a Metro Mirror volume pair is established between two nearby sites (local and intermediate) to protect from local site disasters. The Global Mirror volumes can be located thousands of miles away and can be updated if the original local site has suffered a disaster but has performed failover operations to the intermediate site. In the case of a local-site-only disaster, Metro/Global Mirror can provide zero-data-loss recovery at the remote site as well as at the intermediate site.
- A nearby two-site synchronous copy that can protect from local disasters.
- A longer distance asynchronous copy, at a third site, that can protect from larger scale regional disasters. The third site provides an extra layer of data protection.
The Metro/Global Mirror function provides this combination of synchronous and asynchronous mirroring. Metro/Global Mirror is an extension of Global Mirror, which is based on existing Global Copy (formerly known as PPRC XD) and FlashCopy® functions. Global Mirror running at the intermediate site, using a master storage unit and optional subordinate storage units, internally manages data consistency, removing the need for external software to form consistency groups at the remote site.
Figure 1 shows the three sites that are used in a Metro/Global Mirror configuration. The configuration uses a minimum of three storage units, one each at the local, intermediate, and remote sites. A minimum of four volumes (volume A, volume B, volume C, and volume D) are used in this configuration. (For ease of description, the Metro/Global Mirror configuration is described using these volumes. Some environments can contain hundred or thousands of volumes.)
Data from the A volumes at the local site is synchronously replicated to the 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 are created with the C volumes at the remote site as the FlashCopy source volumes and the D volumes at the remote site as the FlashCopy target volumes, maintaining the consistent disaster recovery volumes using Global Mirror.
As an additional layer of disaster protection if Global Mirror processing were to fail at the remote site, you can use the storage at your intermediate site for a target copy. Setting up Global Mirror between the remote and intermediate sites requires an additional set of FlashCopy volumes at the intermediate site. Then, you can perform failover and restore operations at the remote site using these volumes at the intermediate site (acting as a remote site) to create Global Mirror consistency groups. These volumes, which are referred to as E volumes, would be used as FlashCopy targets for a Global Mirror consistency group.
For Global Mirror processing, one storage unit at the intermediate site is designated as the master storage unit. The master storage unit sends commands over Fibre Channel Protocol (FCP) links and coordinates the consistency group formation process. These links are required for the Global Mirror master storage unit to coordinate the consistency group formation process with the storage units and to communicate the FlashCopy commands to the remote site. All status is relayed back to the master storage unit.