Arbitration For Data Integrity in Serviceguard Clusters, July 2007

Arbitration for Data Integrity in Serviceguard Clusters

Arbitration in Disaster-Tolerant Clusters

Disaster-tolerant clusters are those which are intended to survive the

loss of a data center that contains multiple resources. Examples are an

extended distance cluster where nodes may be separated into different

data centers on one site; metropolitan clusters, where the nodes are

separated into equal-sized groups located a significant distance apart;

and continental clusters, in which the geographically separate data

centers provide a home for entirely separate clusters, complete with

storage devices.

Extended Distance Clusters

In extended distance (campus) clusters, the nodes are divided into two

separate data centers in different buildings, which can be as far as 100

km apart. A dual cluster lock disk may be used for arbitration, with the

two lock disks located in the two different data centers. This affords

protection against the loss of one of the data centers. If a quorum server

is used, it must be in a different location from either of the two data

centers, thus providing additional protection against data center loss.

Similarly, if arbitrator nodes are used, they must be in a different

location from the two data centers.

An extended distance cluster is no different from a standard

Serviceguard cluster except that components are subdivided by data

center. This means that groups of nodes are located in different

buildings, and storage units with mirrored data are placed in separate

facilities as well.

Metropolitan Clusters

Arbitration in a MetroCluster configuration has traditionally followed a

different model than that of the single arbitrator device (originally this

was a lock disk). Because a MetroCluster configuration contains two

distinct data centers at some distance from one another, the main

protection has been against a partition that separates the two data

centers into equal-sized groups of nodes. A lock disk is not possible in

this type of cluster, since metropolitan clusters use a specialized data

replication method rather than LVM mirroring. Since LVM is not used