A-25
Theory of Controller Operation
Note:
When Clear Configuration is selected in the Flash Utility
menu, both the local Flash configuration and the other controller’s
mirrored configuration image are cleared. Also, there is an option
(J) in the Flash Utility menu that can be used to tell a booting
controller to temporarily ignore the mirrored configuration. This
can be used, if for whatever reason, the mirrored configuration
should not be used by a booting controller. This is primarily a
safeguard, because a retrieved mirrored configuration is checked
extensively before it is used.
Clustering
Minimizing Downtime for Maximum Data Availability
So-called open systems, such as Windows NT 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 percent uptime. Adding a RAID subsystem can increase the
uptime to 99.9 percent. The goal of clustering is 99.99 percent
availability.
Beyond clustering, fault-tolerant systems can provide 99.9999
percent uptime. At the high end, continuous-processing systems
offer virtually 100 percent uptime.
Although the increase from 99.5 percent to 99.99 percent
availability may seem insignificantly small, it adds up in terms of
minutes per year of downtime. For example, assuming a 7x24
operation, 99.5 percent uptime translates into 2,628 minutes, or
DuraStor IUG - Online.book Page 25 Friday, July 19, 2002 3:06 PM
