резерв мощности на покрытие несбалансированности нагрузок разного вида

Stranded capacity
Stranded capacity is capacity that cannot be utilized by IT loads due to the design or configuration of the system. The presence of stranded capacity indicates an imbalance between two or more of the following capacities:

• Floor and rack space
• Power
• Power distribution
• Cooling
• Cooling distribution

A specific IT device requires sufficient capacity of all of the five above elements. Yet these elements are almost never available in an exact balance of capacity to match a specific IT load. Invariably, there are locations with rack space but without available cooling, or spaces with available power but with no available rack space. Capacity of one type that cannot be used because one of the other four capacities listed above has been used to its maximum capacity is called stranded capacity. Stranded capacity is undesirable and can seriously limit the performance of a data center. Unfortunately, most data centers have significant stranded capacity issues, including the following common examples:

• An air conditioner has sufficient capacity but inadequate air distribution to the IT load
• A PDU has sufficient capacity but no available breaker positions
• Floor space is available but there is no remaining power
• Air conditioners are in the wrong location
• Some PDUs are overloaded while others are lightly loaded
• Some areas are overheated while others are cold

Depending on the situation and the architecture of the power and cooling system, it might be impossible to utilize stranded capacity or it might be that only minor investments are needed to free stranded capacity so that it can be effectively used. By definition, utilizing stranded capacity comes at a cost. It is often necessary to take down part of the installation or install new power and cooling components.
Stranded capacity is a very frustrating capacity management problem for data center operators because it is very hard to explain to users or management that a data center with 1 MW of installed power and cooling capacity can’t cool the new blade servers when it is only operating at 200 kW of total load.
An effective capacity management system not only identifies and highlights stranded capacity, but also helps customers avoid creating it in the first place.

[APC]