Chapter 1: Cabling Concepts and Best Practices 5
Failover Failover is an automatic operation that switches from a failed component or failing
component to an operational component. In the case of a Redundant Array of
Independent Disks (RAID) controller failover, an operational controller takes over the
ownership of volumes. The operational controller processes I/O from the host in place
of the failing component or failed controller. Controller failover is possible only in
controller trays or in controller-drive trays that contain two controllers.
In a system in which the alternate path software tool is installed on the host, the data
paths through the failed HBA are replaced by data paths through the surviving HBA.
For more information, refer to the topics under Failover or to the corresponding PDF
on the SANtricity ES Storage Manager Installation DVD.
Redundant and
Non-Redundant
The term redundant indicates the existence of something more than what is essential
to accomplish a task. In storage area network technology, redundancy means that
essential services are implemented using duplicated components in such a way that if
one component fails an alternative component continues to provide the service. This
redundancy ensures the availability of data in case a component fails.
In most RAID systems, most of the components are redundant, but that the system
might not be fully redundant. In other words, there might be one or two components
whose individual failures would cause loss of access to data. Therefore, a fully
redundant system duplicates all components and is configured to make sure that the
duplicate components can be accessed in case of a failure. The manner in which the
system is connected is an essential component of creating a successfully configured
redundant system.
Single Point of
Failure
Any component or path that is not duplicated (redundant) or whose failure can cause
loss of data access is called a potential single point of failure. The cabling scenarios in
this document note the components that present a potential single point of failure.
Choose a cabling topology that does not create a single point of failure.
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