Designing Disaster Recovery Clusters using Metroclusters and Continentalclusters, Reprinted October 2011 (5900-1881)

Figure 1 Two Data Centers and Third Location with Arbitrators
A disk array can be the main disk array for one set of packages and the remote disk array for
another. In Figure 1, the P9000 and XP disk array in data center A is the main or primary disk
array for packages A and B, and the remote or secondary disk array for packages C and D in
data center B. For packages A and B, data is written to PVOLs on the array in Data Center A and
replicated to SVOLs on the array in Data Center B. Likewise the P9000 and XP disk array in Data
Center B is the primary or main disk array for packages C and D, and the secondary or remote
for packages A and B. For packages C and D, data is written to PVOLs on the disk array in Data
Center B and replicated to SVOLs in Data Center A.
Arbitrators provide functionality like that of the cluster lock disk, and act as tie-breakers for a cluster
quorum in case all the nodes in one data center go down at the same time. Cluster lock devices
are not supported in a Metrocluster architecture because cluster locks cannot be maintained across
the replication link, such as Continuous Access, SRDF, or 3PAR Remote Copy.
Arbitrators are fully functioning systems that are members of the cluster, and are not usually
physically connected to the disk arrays. A Quorum Server is an alternative form of cluster arbitration
that uses a server program to determine cluster membership rather than a cluster lock disk or a
Serviceguard Arbitration Node.
In the Metrocluster environment, the same number of systems must be present in each of the two
data centers (Data Center A and Data Center B) whose systems are connected to the disk arrays.
However, when using two arbitrator nodes in third location, you can have one node in one
datacenter and two nodes in another datacenter.
Table 3 lists the allowable number of nodes at each main data center and the third location, up
to a 16-node maximum cluster size.
Designing a Disaster Recovery Architecture for use with Metrocluster Products 25