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DuraStor 412R/6320SS/7320SS Installation and User's Guide
more than 43 hours of downtime per year. In contrast, 99.99 percent
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 percent 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.
High availability is not the only benefit of clustering. In some cases,
users may see advantages in the areas of performance, scalability,
and manageability. In reality, you can expect a 1.6x (80 percent
efficiency) to 1.8x (90 percent efficiency) performance increase as
you go from one node to two nodes. Going from one node to a four
node cluster generally yields a 2.5x or 3x performance boost.
However, the cluster performance is application dependent. For
example, READ operations may yield a 1.8x performance increase
going from one to two nodes, but in a WRITE intensive application,
you may only see a 1.4-1.6x improvement.
Although clusters seem to be relatively simple, they involve
complex technology that can be implemented in a variety of ways.
The number of nodes supported and type of interconnection used,
and a number of other features differentiate cluster
implementations. One area of implementation is the manner in
which distributed lock manager is implemented. Some perform
this at the user level and others in the kernel, with the latter
enhancing performance.
In addition to the differing features you should consider if the
cluster:
has the ability to hot load new nodes without bringing down
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the whole cluster?
provides automatic or manual failover?
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loads balance?
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uses a journalized file system?
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provides a fast cluster failover?
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A-26
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