IBM System storage DS6000 Series Redbook page 453

Performance mismatch (running applications)
Suppose a disaster or failure occurs and applications failover to the secondary (or recovery)
site and are running using the secondary Storage Units. If the secondary Storage Unit, the
DS6000, is less capable (in performance) than the primary Storage Unit, it is likely that you
will not be able to complete primary business applications in the required or expected time
frame.
z/OS Global Mirror-capable local (or primary) Storage Units
Suppose a disaster or failure occurs in a z/OS Global Mirror environment and applications
failover to the secondary site and are running at the secondary site on the secondary Storage
Units. Later, after the primary site has been repaired and is ready to resume as the primary
site, the secondary Storage Unit can then use z/OS Global Mirror to failback to the primary
site. However, for the failover and failback operations to work successfully, the secondary
Storage Unit must be a Global Mirror-capable primary Storage Unit, which means it must be
capable of being a Global Mirror primary Storage Unit. The DS6000 does not have the
appropriate microcode functionality to be a Global Mirror-capable primary Storage Unit and,
therefore, cannot be used to failback to the primary site.
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Chapter 28. Interoperability between DS6000 and DS8000