**5.1. Architectures**

Data center power infrastructure is responsible for providing uninterrupted, conditioned power at correct voltage and frequency to the IT equipments. Figure 19 (a) depicts a real-world power infrastructure. From the utility feed (i.e., AC Source), typically, the power goes through voltage panels, uninterruptible power supply (UPS) units, power distribution units (PDUs) (composed of transformers and electrical subpanels), junction boxes, and, finally, to rack PDUs (rack power distribution units). The power infrastructure fails (and, thus, the system) whenever both paths depicted in Figure 19 are not able to provide the power demanded (500 kW) by the IT components (50 racks). The reader should assume a path as a set of redundant interconnected components inside the power infrastructure. Another architecture is analyzed with an additional electricity generator (Figure 19 (b)) for supporting the system when both AC sources are not operational.

**Figure 19.** Data Center Power Architectures.

**Figure 21.** SPN of Architectures A2.

**Figure 22.** RBD of Architectures A2.

and are shown in Table 3.

**5.3. Results**

In architecture A2, the generator is only activated when both AC sources are not available. Therefore, a model that deal with dependencies must be adopted. Figure 21 shows the SPN model considering cold standby redundance to represent the subsystem composed of generator and two AC sources. Besides, we assume that UPS' batteries support the system during the generator activation. The reliability or availability is computed by the probability

A Petri Net-Based Approach to the Quanti cation of Data Center Dependability 333

The other components of the architecture A2 are modeled using RBD as shown in Figure 22. Once obtained the results of both models (RBD and the SPN model with dependencies), a RBD model with two blocks (considering the results of those models) in a serial arrangement is created. The RBD evaluation provides the dependability results of the architecture A2 system. The adopted MTTF and MTTR values for the power devices were obtained from [21] [29] [19]

Figure 23 depicts a graphical comparison between the reliability results (in number of 9's) of those two data center power architectures. The respective number of nines (-*log*[1 - A/100]) and the period of 8760 hours (1 year) are adopted. As the reader should note, the reliability of both architectures decreases when the time increases. Besides, it is also possible to notice that

*P*{#*ACSource*1\_*ON* = OR #*ACSource*2\_*ON* = 1 OR #*Generator*\_*ON* = 1}.
