| IBM's tool for building UNIX-based, mission-critical computing platforms is the HACMP for AIX software. The HACMP for AIX software ensures that critical resources are available for processing. HACMP for AIX has two major components : high availability (HA) and cluster multi-processing (CMP). Until recently, the only avenue for achieving high availability in the UNIX realm was through fault-tolerant technology. Fault tolerance relies on specialized hardware to detect a hardware fault and instantaneously switch to a redundant hardware component ”whether the failed component is a processor, memory board, power supply, I/O subsystem, or storage subsystem. Although this cutover is apparently seamless and offers non-stop service, a high premium is paid in both hardware cost and performance because the redundant components do no processing. More importantly, the fault-tolerant model does not address software failures, by far the most common reason for down-time. High availability views availability not as a series of replicated physical components, but rather as a set of system-wide, shared resources that cooperate to guarantee essential services. High availability combines software with industry-standard hardware to minimize down-time by quickly restoring essential services when a system, component, or application fails. While not instantaneous, services are restored rapidly , often in less than a minute. The difference between fault tolerance and high availability, then, is this: A fault-tolerant environment has no service interruption, while a highly available environment has a minimal service interruption. Many sites are willing to absorb a small amount of down-time with high availability rather than pay the much higher cost of providing fault tolerance. Additionally, in most highly available configurations, the backup processors are available for use during normal operation. "Cluster multi-processing" is a group of loosely coupled machines networked together, sharing disk resources. In a cluster, multiple server machines cooperate to provide a set of services or resources to clients . Clustering two or more servers to back up critical applications is a cost-effective high-availability option. You can use more of your site's computing power while ensuring that critical applications resume operations after a minimal interruption caused by a hardware or software failure. Cluster multi-processing also provides a gradual, scalable growth path . It is easy to add a processor to the cluster to share the growing workload. You can also upgrade one or more of the processors in the cluster to a more powerful model. If you were using a fault-tolerant strategy, you must add two processors, one as a redundant backup that does no processing during normal operations. The latest version of HACMP software (AIX HACMP version 5.1) optimizes availability by allowing for the "dynamic reconfiguration" of running clusters. Most routine cluster maintenance tasks , such as adding or removing a node or changing the priority of nodes participating in a resource group, can be applied to an active cluster without stopping and restarting cluster services. HACMP software allows you to combine these components into a wide range of cluster configurations on RS/6000 systems, providing you with flexibility in building a cluster that meets your processing requirements. Nodes form the core of an HACMP cluster. A node is a processor that runs both AIX and the HACMP for AIX software. The HACMP for AIX software supports pSeries (or RS/6000) uniprocessor and symmetric multiprocessor (SMP) systems and the Scalable POWERParallel processor (SP) systems as cluster nodes. To the HACMP for AIX software, an SMP system looks just like a uniprocessor . SMP systems provide a cost-effective way to increase cluster throughput. Each node in the cluster can be a large SMP machine, extending an HACMP cluster far beyond the limits of a single system and allowing thousands of clients to connect to a single database. In an HACMP cluster, each node is identified by a unique name . A node may own a set of resources ”disks, volume groups, file systems, networks, network addresses, and applications. Typically, a node runs a server or "back end" application that accesses data on the shared external disks. HACMP software supports from two to eight nodes in a cluster, depending on the disk technology used for the shared external disks. HACMP/ES clusters can have up to 32 nodes in a cluster. |
