Rackable Systems OmniStor 4700S Series, User Manual

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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.