More Common Scenario
The more common scenario is two rack-level PDUs and that (hopefully) each of the server's PS cords are plugged into a different PDU. This creates a "sense" of redundancy for most administrators. In reality, this is where the hidden exposure to power problems starts.Let's look at how and why this seemingly simple and common practice is the potential cause of power failures in the data center. In most cases, the dual supplies will share the server load at approximately 50 percent each, when both supplies are active. However, if either PS fails or has lost input power, the remaining PS must draw 100 percent of the power required. Here is where the problem materializes. Servers are normally installed and operated with both rack-level PDUs available. Typically each PS would only draw 50 percent of the server's power requirement. Normally, the PDU load is less (again hopefully) than the trip value of the circuit breaker that protects it. In fact, even if the PDU has a current meter, most administrators would think they have the capacity to add more servers if they are "only" at a 60 percent power level. At 60 percent, the PDUs are overloaded and no one even realizes it! Here is why: If a server or blade server experiences a PS failure, then 100 percent of the power will be drawn from the remaining PS and therefore from the PDU that supports it. This means that at a 60 percent load (at normal conditions), 120 percent of the PDU's power rating will be put on the remaining PDU and the circuit breaker will trip on the PDU (or branch breaker), shutting down all equipment in that rack. This is a classic cascade failure. The same scenario would hold true if another server or other equipment was added that overloaded the PDU load past the tripping point of either PDU.