IBM N Series, Hardware Manual

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IBM System Storage N series Hardware Guide
Figure 8-2 Logical view of MetroCluster SyncMirror
Geographical separation of N series nodes is implemented by physically separating
controllers and storage, which creates two MetroCluster halves. For distances under 500 m
(campus distances), long cables are used to create Stretch MetroCluster configurations.
For distances more than 500 m but less than 100 km (metro distances), a fabric is
implemented across the two locations, which creates a Fabric MetroCluster configuration.
The Cluster_Remote license provides features that enable the administrator to declare a site
disaster and start a site failover by using a single command. The
cf forcetakeover -d
command starts a takeover of the local partner, even in the absence of a quorum of partner
mailbox disks. This command gives the administrator the ability to declare a site-specific
disaster and have one node take over its partner’s identity without a quorum of disks.
The following requirements must be in place to enable takeover in a site disaster:
򐂰 Root volumes of both storage systems must be synchronously mirrored.
򐂰 Only synchronously mirrored aggregates are available during a site disaster.
Administrator intervention, that is, running the forcetakeover command, is required as a
safety precaution against a split brain scenario.
Attention: Site-specific disasters are not the same as a normal cluster failover.