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Clustering is a fault-tolerant server technology that provides availability and scalability. It groups servers and shared resources into a single system that can provide immunity from faults. Improved performance is a byproduct of such a system. Clients interact with clusters of servers as if they are a single system. Should one of the servers fail in the cluster, the other servers can take over its load.

An interesting aspect of clustering is that it can provide "four nines" of availability (99.99 percent uptime), which translates to 53 minutes of downtime per year. A complete description of availability is under "Fault Tolerance and High Availability."

Before discussing clustering, it is important to put the range of network server/storage devices and configurations into context. Here are the typical arrangements of servers and storage devices on enterprise networks and Web sites on the Internet:

• SMP (symmetric multiprocessing) systems    A single system with multiple processors, multiple power supplies, network interface cards, and multiple storage devices that provide "local" fault tolerance (if one processor, power supply, or interface fails, the others remain operational), but not disaster tolerance (flood or fire). These systems can provide scalable performance, but do not provide an ideal scalable storage environment.


• LAN-based server configurations    This is the traditional LAN or wide area network in which servers are attached to the LAN at various locations in a building or over a MAN or WAN. Replication can be used to copy data to different locations in the same building or the extended network. This provides protection against local disasters such as fires and equipment failures. However, the systems are loosely coupled and do not provide the performance and management benefits of the systems discussed next.


• Clustering    A clustered system is a set of servers and attached storage devices that are in the same location and in a configuration that provides fault tolerance against the failure of any one device. All of the servers appear as a single server to users. Requests are balanced between the servers, either by external load-balancing devices or by software provided in the operating systems. All servers have access to all the storage devices, so if one server goes down, access is still available to hard drives through other servers in the cluster.


• SAN (Storage Area Network)    SANs basically put storage devices on their own high-speed network, typically Fibre Channel. By having all the devices attached to the network, any storage device becomes easily accessible. The switching nature of the network allows servers to make direct connections to any device. See "SANs (Storage Area Networks)" and "Fibre Channel." SANs are often created with multiple clustered systems.


• NAS (Network Attached Storage)    A NAS is now commonly referred to as a network appliance. The basic concept is to remove the server altogether and attach storage directly to the network, thus reducing cost and simplifying management. While a SAN is typically a system defined for enterprise use, a NAS is often installed at the department level. This concept relies on the fact that with standard document formats and XML, proprietary file formats used by servers are no longer needed and the server can be removed altogether. According to George Gilder, NASs are all about removing storage from the enslavement of server operating systems. See "NAS (Network Attached Storage)."

Note that clusters, SANs, and NASs all separate storage from servers, thus allowing any server to access any storage device. In addition, access to storage is not dependent on any one server since clients can get at storage through any server (or directly, in the case of NAS).

A simple clustering solution is pictured on the left in Figure C-20. In this case, two servers share the same hard disks. A cluster may also consist of more than two servers, as shown on the right in Figure C-20. The latter is often established to provide additional performance. In fact, a multicluster system is designed to allow scaling as well as fault tolerance. Additional servers are added to the cluster as processing requirements grow.