Chapter 10. Copy services 585
10.8 Consistency protection for Remote and Global mirror
Prior to V7.8.1, Metro Mirror and regular Global Mirror relationships and consistency groups
stop when one of the following events occurs:
The link between systems goes down
A secondary volume goes offline
Consistency of the secondary volume would be lost during resynchronization, so the
relationship is automatically stopped and a 1720 error is raised, which would require the user
to restart the relationship manually.
Global Mirror with Change Volumes (GMCV) relationships use a secondary change volume to
retain a consistent copy during resync and automatically restart when they can.
From V7.8.1, Metro Mirror and regular Global Mirror also behave like GMCV relationship if a
secondary change volume is configured which will do the following:
Makes Metro Mirror and Global Mirror more suited to links with intermittent connectivity
and IP replication
Stop as before if no secondary change volume configured
The consistency protection mechanism for metro mirror and regular global mirror uses
change volumes and has following characteristics:
It is a tweak to existing Metro Mirror and Global Mirror copy types using technology
already in Global Mirror with Change Volumes
Does not need FlashCopy license
Uses two FlashCopy maps per relationship per system (so maximum of 2500 relationships
on a 10k volume-capable system)
Supported on all systems that can have remote mirroring license
Requires both participating systems to be at V7.8.1 or later
Consistency protection for metro or regular Global mirror can be enabled by configuring a
secondary change volume and no further configuration needed to enable this behavior. All
relationships in a consistency group have to be so configured for this behavior to work on any
relationship in the consistency group.
Table 10-11 describes the events and the expected behavior when consistency protection
mechanism has been enabled for metro mirror and regular global mirror.
Table 10-11 Events and expected behavior
Event Expected behavior for the relationship
Link down or secondary volume offline
Relationship is consistent_synchronized
(started and in sync)
Retain the secondary consistent copy
Prepare for resynchronization
Go to the consistent_copying state
Automatically resume replication as and when
possible
Go back to consistent_synchronized when
complete
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