IBM N Series, Hardware Manual

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Chapter 7. Highly Available controller pairs
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򐂰 If you are using third-party storage, paths to an array LUN must be redundant.
License requirements
The following licenses must be enabled on both nodes:
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cf
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syncmirror_local
7.2.3 Stretched MetroCluster
Stretch MetroCluster includes the following characteristics:
򐂰 Stretch MetroClusters provide data mirroring and the ability to start a failover if an entire
site becomes lost or unavailable.
򐂰 Stretch MetroClusters provide two complete copies of the specified data volumes or file
systems that you indicated as being mirrored volumes or file systems in an HA pair.
򐂰
Data volume copies are called plexes, and are continually and synchronously updated
every time Data ONTAP writes data to the disks.
򐂰 Plexes are physically separated from each other across separate groupings of disks.
򐂰 The Stretch MetroCluster nodes can be physically distant from each other (up to
500 meters).
Figure 7-4 shows a simplified Stretch MetroCluster.
Figure 7-4 Simplified Stretch MetroCluster
Remember: Unlike mirrored HA pairs, MetroClusters provide the capability to force a
failover when an entire node (including the controllers and storage) is unavailable.