Cloud Computing

The National Institute of Standards and Technology (NIST) has dened Cloud Com-puting as "a model for enabling ubiquitous, convenient, on-demand network access to a shared pool of congurable computing resources [...] that can be rapidly provi-sioned and released with minimal management eort or service provider interaction"

where "computing resources" refers both to the IT infrastructure (servers, storage, and networks components) and the abstract application layer (virtualization, inter-faces, and software) [28]. The wide range of services provided by Cloud Computing has been divided into three main cloud service delivery models (see Figure 2):

• Software-as-a-Service (SaaS): an application running on the Cloud and ac-cessible by the end user through an interface like a web browser.

• Platform-as-a-Service (PaaS): a congurable platform integrating both hard-ware and softhard-ware tools for web application development.

• Infrastructure-as-a-Service (IaaS): a virtualized pool of computing resources physically located in the cloud and manageable over the Internet.

Figure 2. Cloud service delivery models: SaaS, PaaS, and IaaS, based on [29].

Yet, the list is not exhaustive as many other "XaaS" have been coined such as STorage-as-a-Service (StaaS) and Network-as-a-Service (NaaS) [30].

The Cloud Computing IT infrastructure is amassed in large data centers. It needs to be managed, maintained, upgraded, and consumes huge amount of energy every day, leading to major costs for its owner. Therefore, the deployment of a Cloud infrastructure is a strategic move that need to be evaluated beforehand. Currently, four Cloud infrastructure models have been dened (see Figure 3):

• Private Cloud: the infrastructure is used by a single organization but can be managed and/or owned by a third party.

• Public Cloud: the infrastructure is available to the general public over the Internet oering limited security and variable performances.

• Community Cloud: the infrastructure is shared by several organizations but commonly managed internally or by a third party.

• Hybrid Cloud: the infrastructure is a combination of Cloud models that remains a unique entity, oering more exibility at the expense of complexity.

Each model has dierent degree of security, complexity and management. Thus, the model choice will be done according to requirements or needs of the organization.

Figure 3. Deployment models of Cloud solutions.

Regardless of the model, a Cloud Computing solution provides a great trade-o between convenience, cost and exibility. From the hardware side, [31] identies three new facets conferred by Cloud Computing:

• appearance of unlimited available resources procured by the rapid elasticity of the system outward and inward;

• opportunity to scale up or down your infrastructure dynamically and auto-matically, according to your personal or business needs over the time, and consequently to pay for the real usage ("On-demand self-service");

• possibility to access "ready-to-use" computing environment and eliminating the up-front commitment by Cloud users.

Such a degree of convenience is achieved through virtualization of hardware resources into multiple virtual machines and virtual storage. While virtualization improves the scalability of the cloud system and reduces the number of physical equipment, it requires powerful resources to not aect the performances of the system since one physical machine might handle multiple virtual ones.

At the storage level, virtualization backwards specic Direct Attached Storage (DAS) resources to a pool of storage, accessible through an internal network (Network At-tached Storage (NAS) and Storage Arena Network (SAN)).

Whereas the storage technologies have changed dramatically in the past few years, there is today a "performance gap" [32] between the CPUs speed of servers and the related memory or storage subsystem. Due to dierent evolution history, stor-age elements became a bottleneck in the computing system, aecting performances when data is requested by processor tasks. The Solid State Drive (SSD) elimi-nates the binding mechanical parts of Hard Disk Drives (HDDs), thus reduces the overall access time and improves its performances. Additionally, this technology is more reliable and consumes less energy, which makes the SSD technology "greener".

However, its cost-per-bit is still signicantly higher than HDD, which makes SDD solution cost-prohibitive and limits its utilization to critical I/O applications. Thus, HDDs storage resources are still widely present in modern data centers.

Concerning energy consumption and sustainability, Cloud Computing has a signi-cant impact on the environment. According to [3335], running physical equipment represents about 40% to 55% of the energy bill in a data center. Then, 30% to 45%

is used to cool the equipment due to thermal characteristic of electronic circuitry and 10% to 15% is consumed by Uninterruptible Power Supplies (UPSs), lighting and other. A report from the Natural Resources Defense Council (NRDC) [36] eval-uated in 2013 that the U.S. data centers were using the equivalence of 34 power plants, each of them generating 500 megawatts of electricity. The resulting CO2

emissions were close to 100 million Tons. Recently, Big companies like Facebook, Google and Apple decided to build their data centers in the Nord part of Europe, for dierent reasons including electricity price, social stability but mainly for the adequate climate, enabling to use outside air to cool down IT equipment and reduce the overall data center energy consumption (see Figure 4).

Figure 4. Free Cooling in Facebook data center, Luleå (Sweden) [37].

However, even though energy consumption is an important factor of the Cloud Computing environmental impact, it is not proportional to CO2 emissions. Cloud providers are looking for alternative source of energy to reduce their carbon dioxide emanations. As a good example, all the equipment inside Facebook Luleå data center is powered by 100% renewable energy locally generated by hydroelectric power plans. In another way, Google is investing in carbon osets projects and buy clean power from specic producer in order to balance emissions from their data centers.