2.5 Cluster - Closely Coupled


2.5 Cluster – Closely Coupled

The TruCluster Server product provides for the close interaction of multiple systems. A cluster is generally considered to be "closely coupled" in that the systems have an awareness of each other but do not access the system code in common memory (as tightly coupled systems would) and do not rely on a traditional network for communications (as a loosely coupled arrangement would be). It may help to think of a cluster as having the characteristics of both loosely coupled and tightly coupled systems. It is like a loosely coupled system because it is easy to add more horsepower by adding a new member to the cluster (similar to adding another CPU in a tightly coupled arrangement), and if one member goes down, you're not necessarily sunk. It also resembles a tightly coupled system in that the machines directly share storage, file system, and hostname (sort of – each member will have a unique name, but there will also be a name for the entire cluster) and act as one virtual entity (despite the fact that each system has its own physical memory).

Figure 2-7 shows two members in a cluster with three connections between the members: cluster interconnect, shared bus, and network interconnect. At least one cluster-specific interconnect mechanism (Memory Channel or LAN[3]) must be in place to form a cluster. Note that the network interconnect shown below is used for network traffic such as NFS, while the cluster interconnect is used for intra-cluster communication. The shared bus is common and highly recommended but optional.

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Figure 2-7: Two-Member Cluster

A cluster is coupled in one of two ways. It may either use specialized hardware such as Memory Channel or use a LAN-based interconnect for a slower but less expensive communications alternative. Cluster members must be aware of the state of the cluster. Communication and synchronization must be achieved in order to determine the viability of the cluster.

You may have several questions by now, including: "Should a cluster continue operations if one member of a three member cluster goes down?," "What happens to the applications that were running on the failed member?," "How do the other members decide whether to continue?," "How does the cluster appear to other nodes in the network?," and "How is I/O coordinated between cluster members sharing a common bus and I/O access?" These questions will be addressed in the following sections. The issues of kernel mode versus user mode, interrupts, synchronization, and cooperation among applications will all be revisited, only this time within the context of a cluster.

[3]Local Area Network




TruCluster Server Handbook
TruCluster Server Handbook (HP Technologies)
ISBN: 1555582591
EAN: 2147483647
Year: 2005
Pages: 273

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