Cluster Membership Monitor

The Cluster Membership Monitor (CMM) is a distributed set of agents, one per cluster member. The agents exchange messages over the cluster interconnect to:

• Enforce a consistent membership view on all nodes (quorum)

• Drive synchronized reconfiguration in response to membership changes, using registered callbacks

• Handle cluster partitioning (split brain, amnesia)

• Ensure full connectivity among all cluster members

Unlike previous Sun Cluster software releases, CMM runs entirely in the kernel.

Cluster Membership

The main function of the CMM is to establish cluster-wide agreement on the set of nodes that participates in the cluster at any given time. This constraint is called the cluster membership.

To determine cluster membership, and ultimately, ensure data integrity, the CMM:

• Accounts for a change in cluster membership, such as a node joining or leaving the cluster

• Ensures that a “bad” node leaves the cluster

• Ensures that a “bad” node stays out of the cluster until it is repaired

• Prevents the cluster from partitioning itself into subsets of nodes

See Quorum and Quorum Devices for more information on how the cluster protects itself from partitioning into multiple separate clusters.

Cluster Membership Monitor Reconfiguration

To ensure that data is kept safe from corruption, all nodes must reach a consistent agreement on the cluster membership. When necessary, the CMM coordinates a cluster reconfiguration of cluster services (applications) in response to a failure.

The CMM receives information about connectivity to other nodes from the cluster transport layer. The CMM uses the cluster interconnect to exchange state information during a reconfiguration.

After detecting a change in cluster membership, the CMM performs a synchronized configuration of the cluster, where cluster resources might be redistributed based on the new membership of the cluster.

Failfast Mechanism

If the CMM detects a critical problem with a node, it calls upon the cluster framework to forcibly shut down (panic) the node and to remove it from the cluster membership. The mechanism by which this occurs is called failfast. Failfast will cause a node to shut down in two ways.

• If a node leaves the cluster and then attempts to start a new cluster without having quorum, it is “fenced” from accessing the shared disks. See Failure Fencing for details on this use of failfast.

• If one or more cluster-specific daemons die (clexecd, rpc.pmfd, rgmd, or rpc.ed) the failure is detected by the CMM and the node panics.

panic[cpu0]/thread=40e60: Failfast: Aborting because "pmfd" died 35 seconds ago.
409b8 cl_runtime:__0FZsc_syslog_msg_log_no_argsPviTCPCcTB+48 (70f900, 30, 70df54, 407acc, 0)
%l0-7: 1006c80 000000a 000000a 10093bc 406d3c80 7110340 0000000 4001 fbf0

After the panic, the node might reboot and attempt to rejoin the cluster or stay at the OpenBoot^TM PROM (OBP) prompt. The action taken is determined by the setting of the auto-boot? parameter in the OBP.