Metro/Global Copy is a cascaded PPRC solution that enables you to combine two PPRC pairs, synchronous PPRC and PPRC-XD, which operates asynchronously, to create a low-cost, long-distance, tier four data loss disaster recovery configuration. In a cascading PPRC configuration, a single volume is defined as both a primary and a secondary, combining three volumes into two PPRC pairs.
In Figure 1, we see two PPRC pairs, local pair A-B, and remote pair B-C. Volume B acts in two roles simultaneously. Volume B is the secondary volume of the local pair and the primary volume of the remote pair. As in any PPRC relationship, volumes A and B as well as volumes B and C must be in different logical subsystems (in the same or different ESS). There is a minimum of two different ESSs and a maximum of three different ESSs in a cascaded PPRC relationship. In most cases, volumes A, B, and C will be located in different sites: a local site, an intermediate, and a remote site.
Setting up Metro/Global Copy explains how to configure Metro/Global Copy.