IBM DS8000 User Manual page 147

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 11 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.
Local Site
Metro Mirror
A
Figure 11. Metro/Global Mirror configuration
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.
Intermediate Site
Global Copy
B
E
Metro Global Mirror
Chapter 8. Optional feature codes for the DS8000
Remote Site
Global
Mirror
FlashCopy
C D
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