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## 10.8 Consistency protection for Remote and Global mirror 

Before V7.8.1, Metro Mirror and regular Global Mirror relationships and consistency groups stop when one of the following events occurs: 

GLYPH<SM590000> The link between systems goes down 

GLYPH<SM590000> A secondary volume goes offline 

Consistency of the secondary volume is lost during resynchronization, so the relationship is automatically stopped and a 1720 error is raised, which requires 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 does the following: 

GLYPH<SM590000> Makes Metro Mirror and Global Mirror more suited to links with intermittent connectivity and IP replication 

GLYPH<SM590000> 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: 

GLYPH<SM590000> It is a tweak to Metro Mirror and Global Mirror copy types by using technology that is in Global Mirror with Change Volumes 

GLYPH<SM590000> Does not need FlashCopy license 

GLYPH<SM590000> Uses two FlashCopy maps per relationship per system (so maximum of 2500 relationships on a 10k volume-capable system) 

GLYPH<SM590000> Supported on all systems that can have remote mirroring license 

GLYPH<SM590000> 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 is needed to enable this behavior. All relationships in a consistency group must be so configured for this behavior to work on any relationship in the consistency group. 

Table 10-11 lists the events and the expected behavior when consistency protection mechanism is enabled for metro mirror and regular global mirror. 

<img src='content_image/977838.jpg'>

