IBM N Series, Hardware Manual

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IBM System Storage N series Hardware Guide
A Stretch MetroCluster can be cabled to be redundant or non-redundant, and aggregates can
be mirrored or unmirrored. Cabling for Stretch MetroCluster follows the same rules as for a
standard HA pair. The main difference is that a Stretch MetroCluster spans over two sites with
a maximum distance of up to 500 meters.
A MetroCluster provides the
cf forcetakeover -d
command, which gives a single command
to start a failover if an entire site becomes lost or unavailable. If a disaster occurs at one of the
node locations, your data survives on the other node. In addition, it can be served by that
node while you address the issue or rebuild the configuration.
In a site disaster, unmirrored data cannot be retrieved from the failing site. For the surviving
site to do a successful takeover, the root volume must be mirrored.
Setup requirements and restrictions for stretched MetroCluster
You must follow certain requirements and restrictions when you are setting up a new Stretch
MetroCluster configuration.
The restrictions and requirements for stretch MetroClusters include those for a standard HA
pair and those for a mirrored HA pair. The following requirements also apply:
򐂰
SATA and Fibre Channel storage is supported on stretch MetroClusters, but both plexes of
the same aggregate must use the same type of storage.
For example, you cannot mirror a Fibre Channel aggregate with SATA storage.
򐂰 MetroCluster is not supported on the N3300, N3400, and N3600 platforms.
򐂰 The following distance limitations dictate the default speed that you can set:
– If the distance between the nodes is less than 150 meters and you have an 8 Gb FC-VI
adapter, set the default speed to 8 Gb. If you want to increase the distance to
270 meters or 500 meters, you can set the default speed to 4 Gb or 2 Gb.
– If the distance between nodes is 150 - 270 meters and you have an 8 Gb FC-VI
adapter, set the default speed to 4 Gb.
– If the distance between nodes is 270 - 500 meters and you have an 8 Gb FC-VI or 4 Gb
FC-VI adapter, set the default speed to 2 Gb.
򐂰 If you want to convert the stretch MetroCluster configuration to a fabric-attached
MetroCluster configuration, unset the speed of the nodes before conversion. You can
unset the speed by using the
unsetenv command.
License requirements
The following licenses must be enabled on both nodes:
򐂰 cf (cluster failover)
򐂰 syncmirror_local
򐂰 cf_remote
7.2.4 Fabric-attached MetroCluster
Like Stretched MetroClusters, Fabric-attached MetroClusters allow you to mirror data
between sites and to declare a site disaster, with takeover, if an entire site becomes lost or
unavailable.
The main difference from a Stretched MetroCluster is that all connectivity between
controllers, disk shelves, and between the sites is carried over IBM/Brocade Fibre Channel
switches. These are called the
back-end switches .