Rackable Systems OmniStor 4700S Series, Руководство пользователя

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Chapter 2 - Operating Mode and Topologies
Minimizing Downtime for Maximum Data Availability
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Availability figures relate primarily to unplanned downtime. But the advantages of clusters
in terms of planned or scheduled downtime are even more significant.
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.
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% efficiency) to 1.8x (90% 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 differences of features you should consider the following:
Does the cluster:
• have the ability to hot load new nodes without bringing down the whole
cluster?
• provide automatic or manual failover?
• load balance?
• use a journalized file system?
• provide a fast cluster failover?
• allow for the nodes to be geographically located?