# Reducing the Operational Cost of Cloud Data Centers through Renewable Energy
## 3 The System  
This section is devoted to the description of the system architecture. As previously mentioned, the multi-site load management strategy considered in this paper is EcoMultiCloud [2]. EcoMultiCloud consists of a two-layer architecture in which the upper layer is used to exchange information among the different DCs so as to properly drive the distribution of VMs among the sites, while the lower layer is used to allocate the workload within single DCs. At the lower layer, the DCs can use any load management strategy. In this paper, the DCs use the decentralized/self-organizing approach presented in [14] for the consolidation of the workload in a DC. The single DC solution dynamically consolidates VMs to the minimum number of servers and allows the remaining servers to enter low consuming sleep modes. This approach has been proven to be very efficient for the energy consumption reduction of individual data centers. At the lower layer, key decisions regarding the local data center are delegated to the servers, which autonomously decide whether or not to accommodate a VM or trigger a VM migration. At the upper layer, global strategies are implemented by making decisions about workload allocation and inter-DC VM migrations. The decisions are made by combining some general information about single DCs. The hierarchical architecture, organized in two independent layers that exchange some (limited) information, allows upper layer algorithms to be modified independently on the lower layer algorithms. In this way, different strategies can be adopted locally at the DCs. Conversely, improvements of single sites can be implemented without any explicit notification or involvement of the upper layer, i.e., of other DCs, provided that information exchange between layers is maintained. The reference scenario is depicted in Figure 1, which shows the upper and lower layers for two interconnected DCs. At each DC, a data center manager (DCM) runs the algorithms of the upper layer. The DCM integrates the information coming from the lower layer and uses it to implement the functionalities of the upper layer. The DCM communicates with the local manager (LM) and acquires detailed knowledge about the current state of the local DC, for example regarding the usage of host resources and the state of running VMs. Then, the DCM extracts relevant high level information about the state of the DC and transmits this high level information to all the other DCMs (upper layer). The algorithms at the upper layer combine the collected information and make decisions about the distribution of the workload among the DCs. The assignment algorithm is used to decide to which DC a new VM should be assigned. Once the VM is delivered to the target site, the LM runs the lower layer algorithms to assign the VM to a specific host. Figure 1: The EcoMultiCloud approach: the hierarchical two-level architecture in the case of two interconnected data centers. DCM, data center manager; LM, local manager. As shown in Figure 2, the scenario considered in this paper includes four DCs. The DCMs execute the upper layer algorithms by exchanging information among themselves. While the approach requires information exchanges that scale quadratically with the number of DCs, the number of interconnected sites is expected to be small. Moreover, the amount of information to be distributed is tiny, i.e., of the order of a few packets, since it contains only a general description of the status of the DC: only pieces of information such as the load, the PUE and the electricity price need to be exchanged. The periodicity of the information exchange is of the order of an update per one or a few minutes, meaning that less than 1 Kbps of information exchange is needed.