High-Availability Framework

The SunPlex system makes all components on the “path” between users and data highly available, including network interfaces, the applications themselves, the file system, and the multihost disks. In general, a cluster component is highly available if it survives any single (software or hardware) failure in the system.

The following table shows the kinds of SunPlex component failures (both hardware and software) and the kinds of recovery built into the high-availability framework.

Sun Cluster software's high-availability framework detects a node failure quickly and creates a new equivalent server for the framework resources on a remaining node in the cluster. At no time are all framework resources unavailable. Framework resources unaffected by a crashed node are fully available during recovery. Furthermore, framework resources of the failed node become available as soon as they are recovered. A recovered framework resource does not have to wait for all other framework resources to complete their recovery.

Most highly available framework resources are recovered transparently to the applications (data services) using the resource. The semantics of framework resource access are fully preserved across node failure. The applications simply cannot tell that the framework resource server has been moved to another node. Failure of a single node is completely transparent to programs on remaining nodes using the files, devices, and disk volumes attached to this node, as long as an alternative hardware path exists to the disks from another node. An example is the use of multihost disks that have ports to multiple nodes.

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.

Cluster Configuration Repository (CCR)

The Cluster Configuration Repository (CCR) is a private, cluster-wide database for storing information pertaining to the configuration and state of the cluster. The CCR is a distributed database. Each node maintains a complete copy of the database. The CCR ensures that all nodes have a consistent view of the cluster “world.” To avoid corrupting data, each node needs to know the current state of the cluster resources.

The CCR uses a two-phase commit algorithm for updates: An update must complete successfully on all cluster members or the update is rolled back. The CCR uses the cluster interconnect to apply the distributed updates.

Although the CCR consists of text files, never edit the CCR files manually. Each file contains a checksum record to ensure consistency between nodes. Manually updating CCR files can cause a node or the entire cluster to stop functioning.

The CCR relies on the CMM to guarantee that a cluster is running only when quorum is established. The CCR is responsible for verifying data consistency across the cluster, performing recovery as necessary, and facilitating updates to the data.