High-Availability Framework

The Sun Cluster software makes all components on the “path” between users and data highly available, including network interfaces, the applications themselves, the file system, and the multihost devices. 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 Sun Cluster component failures (both hardware and software) and the kinds of recovery that are 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 that are unaffected by a failed 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) that are using the resource. The semantics of framework resource access are fully preserved across node failure. The applications cannot detect that the framework resource server has been moved to another node. Failure of a single node is completely transparent to programs on remaining nodes by using the files, devices, and disk volumes that are available to this node. This transparency exists if an alternative hardware path exists to the disks from another host. An example is the use of multihost devices that have ports to multiple hosts.

Zone Membership

Sun Cluster software also tracks zone membership by detecting when a zone boots up or halts. These changes also trigger a reconfiguration. A reconfiguration can redistribute cluster resources among the nodes in the cluster.

Cluster Membership Monitor

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. In a synchronized configuration, cluster resources might be redistributed, based on the new membership of the cluster.

Failfast Mechanism

The failfast mechanism detects a critical problem on either a global-cluster voting node or global-cluster non-voting node. The action that Sun Cluster takes when failfast detects a problem depends on whether the problem occurs in a voting node or a non-voting node.

If the critical problem is located in a voting node, Sun Cluster forcibly shuts down the node. Sun Cluster then removes the node from cluster membership.

If the critical problem is located in a non-voting node, Sun Cluster reboots that non-voting node.

If a node loses connectivity with other nodes, the node attempts to form a cluster with the nodes with which communication is possible. If that set of nodes does not form a quorum, Sun Cluster software halts the node and “fences” the node from the shared disks, that is, prevents the node from accessing the shared disks.

You can turn off fencing for selected disks or for all disks.

If you turn off fencing under the wrong circumstances, your data can be vulnerable to corruption during application failover. Examine this data corruption possibility carefully when you are considering turning off fencing. If your shared storage device does not support the SCSI protocol, such as a Serial Advanced Technology Attachment (SATA) disk, or if you want to allow access to the cluster's storage from hosts outside the cluster, turn off fencing.

If one or more cluster-specific daemons die, Sun Cluster software declares that a critical problem has occurred. Sun Cluster software runs cluster-specific daemons on both voting nodes and non-voting nodes. If a critical problem occurs, Sun Cluster either shuts down and removes the node or reboots the non-voting node where the problem occurred.

When a cluster-specific daemon that runs on a non-voting node fails, a message similar to the following is displayed on the console.

cl_runtime: NOTICE: Failfast: Aborting because "pmfd" died in zone "zone4" (zone id 3)
35 seconds ago.

When a cluster-specific daemon that runs on a voting node fails and the node panics, a message similar to the following is displayed on the console.

panic[cpu1]/thread=2a10007fcc0: Failfast: Aborting because "pmfd" died in zone "global" (zone id 0)
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 Solaris host might reboot and the node might attempt to rejoin the cluster. Alternatively, if the cluster is composed of SPARC based systems, the host might remain at the OpenBoot PROM (OBP) prompt. The next action of the host is determined by the setting of the auto-boot? parameter. You can set auto-boot? with the eeprom command, at the OpenBoot PROM ok prompt. See the eeprom(1M) man page.

Cluster Configuration Repository (CCR)

The CCR uses a two-phase commit algorithm for updates: An update must be successfully completed 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 yourself. Each file contains a checksum record to ensure consistency between nodes. Updating CCR files yourself can cause a node or the entire cluster to stop working.

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.