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Chapter 2 - Topologies and Operating Modes
A Word about Clustering
Minimizing Downtime for Maximum Data Availability
So-called open systems, such as Windows servers, just don't provide the level of
availability that IS managers are familiar with on mainframes. A partial solution to
this problem is server clustering.
Clusters consist of two or more loosely coupled systems with a shared-disk
subsystem and software that handles failover in the case of a node (host) failure.
In most cases, hardware/software failover is performed automatically and is
transparent to users, although users will experience performance degradation as
processing is shifted to another cluster node. In some cases this failover can
occur in a matter of seconds.
High availability of data and applications is by far the most compelling reason to
go with clustering technology. For example, the accepted rule is that stand-alone
UNIX systems can provide 99.5% uptime. Adding a RAID subsystem can increase
the uptime to 99.9%. The goal of clustering is 99.99% availability.
Beyond clustering, fault-tolerant systems can provide 99.9999% uptime. At the
high end, continuous-processing systems offer virtually 100% uptime.
Although the increase from 99.5% to 99.99% availability may seem insignificantly
small, it adds up in terms of minutes per year of downtime. For example,
assuming a 7x24 operation, 99.5% uptime translates into 2,628 minutes — or
more than 43 hours of downtime per year. In contrast, 99.99% uptime translates
into less than one hour (52 minutes) of downtime per year.
Availability figures relate primarily to unplanned downtime. But the advantages
of clusters in terms of planned or scheduled downtime are even more significant.
If you figure two to sixteen hours per month for a server in a large shop.
Planned downtime requires shutting down stand-alone systems entirely. Result:
100% loss of processing for the duration of the downtime. But, with cluster, you
can shut down one node and off-load the processing to other nodes in the cluster
with no interruption of processing.
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A Word about Clustering
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