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Designing Disaster Tolerant High Availability Clusters, 10th Edition, March 2003 (B7660-90013)

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Building a Metropolitan Cluster Using MetroCluster/CA
Designing a Disaster Tolerant Architecture for use with MetroCluster/CA
Chapter 374
Series disk array can be the main disk array for one set of packages and
the remote disk array for another. In Figure 3-1, the 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 the
array in Data Center A and replicated to SVOLs on the array in Data
Center B. Likewise the 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 of the nodes in one
data center go down at the same time. Cluster lock devices are not used
in the three-data-center architecture because cluster locks cannot be
maintained across the CA link.
Arbitrators are fully functioning systems that are members of the
cluster, and are not usually physically connected to the XP disk arrays. A
Quorum Server is an alternative form of cluster arbitration that uses a
server program to determine cluster membership rather than a cluser
lockdisk or a MC/ServiceGuard Arbitration Node.
Table 3-2 lists the allowable number of nodes at each main data center
and the third location, up to a 16-node maximum cluster size. (Note that
the maximum cluster size for MC/ServiceGuard A.10.12 is 8 nodes.)
Table 3-2 Supported System and Data Center Combinations
Data Center
A
Data Center
B
Data Center
C
ServiceGuard
Version
1 1 1 Arbitrator
Node
A.11.13 or later
11Quorum
Server System
A.11.13 or later
2 1 2 Arbitrator
Nodes
A.11.13 or later
1 2 2 Arbitrator
Nodes
A.11.13 or later