Failover at multiple level in storage networks

Storage networks can be broadly classified into 3 levels and the fail over or Redundancy feature is very important aspect of IT storage business and it is implemented in every interconnecting components in the networks.
Host or Server(s) Level
Network Level
Storage sub-system(s) Level

Fail-Over:
If the data passage path of any of the component in the storage networks fails, the auto-mechanism that provides an alternate path instantly to avoid the data loss is called fail-over.
Example: If a component such as a RAID controller module or a cable fails, or an error occurs on the data path to the preferred RAID controller module, virtual disk ownership is moved to the alternate non preferred RAID controller module for processing. This failure or error is called failover.

Host or Server Level:
Hosts or servers are associated with few interconnect peripherals such as Host Bus Adapter [HBA], Ethernet adapters et al. some common fail-over feature implementations that you see is at the HBA device in the form of Fail-over driver such as MPP or MPIO [Multi Path driver].
Another stage of fail-over implementation could be to the entire host or server in the form of Clustering, Microsoft Clustering or Linux or Solaris or Unix clustering.

Network Level:
At Switches or Routers, we can provide fail-over by connecting one port or path of the servers & storage sub-systems to one switch port & another port or path connected through another switch port. thus we need at the least two switches to handle the Preferred path & Alternative path of data passage.
usually such setups are know as High Availability Fail-over setup.

Sub-system Level:
Within the SAN storage sub-systems, there are multiple components such as RAID Processors or controllers, Hard disks, Drive trays, power supplies et al.
At Hard disk level , RAID & Hot sparing is used in co-operation with RAID processors as fail-over.
At Drive tray or Enclosure level, there will be dual Service Modules that act as fail-over.
At the RAID Processor or Controller level, there will be Dual RAID processors or Controllers that provide fail-over mechanism.

Fail-over is very vast concept and there are plenty of ways to implement it at different components of the storage networks. for more information you can always buzz me on Email.