Arbitration For Data Integrity in Serviceguard Clusters, July 2007

Arbitration for Data Integrity in Serviceguard Clusters

Arbitration in Disaster-Tolerant Clusters

Continental Clusters

There are no special arbitration requirements or configurations for the

separate clusters within a continental cluster. Each cluster must provide

its own arbitration separately, according to the applicable rules for a

standard Serviceguard cluster. In other words, the continental cluster

can employ any supported method of arbitration for its component

clusters.

ContinentalClusters provides semi-automatic failover via commands

which must be issued by a human operator. Between the member

clusters of a continental cluster, the arbitrator is the system

administrator on the recovery site, who must verify that it is appropriate

to issue the cmrecovercl command.

Use of Dual Lock Disks in Extended Distance Clusters

Lock disks are not supported in metropolitan clusters, but they can be

used in an extended distance cluster, which employs mirrored LVM over

a FibreChannel disk link.

NOTE Dual lock LUN configurations are not supported.

For an extended distance cluster, there should be one lock disk in each of

the data centers; all nodes have access to both lock disks via the disk

link. In the event of a failure of one of the data centers, the nodes in the

remaining data center will be able to acquire their local lock disk,

allowing them to successfully reform a new cluster. In a dual lock disk

configuration, there is an extremely slight chance of split-brain in dual

lock disk configuration.

The use of dual locks is as follows in an extended distance cluster:

1. After the loss of heartbeat between the data centers, each sub-cluster

attempts to access the first lock disk, then the second lock disk.

2. If a node in one data center is successful at obtaining the first lock

disk and the disk link between the two data centers is still viable,

then nodes in the second data center will be refused the lock. The

first data center will then be able to obtain the second disk and

re-form the cluster.