nStor NexStor 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?
4700S.book Page 21 Wednesday, April 6, 2005 2:14 PM