Using Redundancy

You can model the redundancy configurations on your system by selecting the type of redundancy you are employing.

Series: A series configuration has no redundancy, and all components are connected in series and all must be operating for the system to successfully function.
Cold Standby: Cold standby is defined as a number of identical components in a redundant configuration. Oftentimes, noted as k-out-of-n redundancy, indicating that a quantity (k) out of a total of (n) components are required for the system to operate. In cold standby, the standby components are not active and will not fail. Once a failure occurs, the standby unit is activated so that the system remains operational.
Hot Standby: Similar to cold standby, however, the standby units are turned on. When a unit fails, the standby unit takes over.
Parallel: In a parallel redundant configuration, all components are operating all the time and only the designated quantity (k) is required to keep the system operating.

For redundant components, you indicate the total number of components, and the number required for operation - the quantity and quantity required.

Also, for redundant components, you may want to consider the switch itself that enables a backup unit when one of the redundant units fails. You may want to account for the switch probability, or the likelihood the switch operates without failure. And specifically, for standby components, you may also wish to include the switch delay, or the amount of time it takes to switch over to a backup unit. Both factor into the reliability and availability metrics of your RBD.