nStor NexStor 4700S Series, Руководство пользователя

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Chapter 2 - Operating Mode and Topologies
A Word about Clustering
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This redirection is known as path failover. The software continuously tries to
access the failed path by issuing a SCSI Test Unit Ready command. A good status
returned indicates the path is repaired and restored to operational status. The
software automatically redirects data back to the primary path and primary disk
object. This restoration of data transfer is known as path failback.
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 storage system 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.
4700S.book Page 20 Wednesday, April 6, 2005 2:14 PM