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Advances, Systems and Applications

Evaluation and design of highly reliable and highly utilized cloud computing systems


Cloud computing paradigm has ushered in the need to provide resources to users in a scalable, flexible, and transparent fashion much like any other utility. This has led to a need for developing evaluation techniques that can provide quantitative measures of reliability of a cloud computing system (CCS) for efficient planning and expansion. This paper presents a new, scalable algorithm based on non-sequential Monte Carlo Simulation (MCS) to evaluate large scale cloud computing system (CCS) reliability, and it develops appropriate performance measures. Also, a new iterative algorithm is proposed and developed that leverages the MCS method for the design of highly reliable and highly utilized CCSs. The combination of these two algorithms allows CCSs to be evaluated by providers and users alike, providing a new method for estimating the parameters of service level agreements (SLAs) and designing CCSs to match those contractual requirements posed in SLAs. Results demonstrate that the proposed methods are effective and applicable to systems at a large scale. Multiple insights are also provided into the nature of CCS reliability and CCS design.


Cloud computing provides a cost-effective means of transparently providing scalable computing resources to match the needs of individual and corporate consumers. Despite the heavy reliance of society on this new technological paradigm, failure and inaccessibility are quickly becoming a major issue. Current reports state that up to $ 285 million yearly have been lost due to such failures with an average of 7.74 hours of unavailability per service per year (about 99.91 % availability) [13]. Despite these outages, rapid adoption of cloud computing has continued for the mission-critical aspects of the private and public sectors, particularly due to the fact that industrial partners are unaware of this issue [2, 3]. This is particularly disconcerting considering President Obama’s $ 20 billion dollar Federal Cloud Computing Strategy and the rapid migration of government organizations like NASA, the Army, the Federal Treasury, Alcohol, Tobacco, and Firearms, the Government Service agency, the Department of Defense, and the Federal Risk and Authorization Management Program to cloud based IT services [4, 5]. Furthermore, companies such as Netflix, IBM, Google, and Yahoo are heavily investing in cloud computing research and infrastructure to enhance the reliability, availability, and security of their own cloud based services [68].

Thus, from the user’s perspective, there is a great need to build a highly available and highly reliable cloud. While cloud providers feel the necessity to provide not only high levels of availability and reliability to meet quality-of-service (QoS) requirements and service level agreements (SLAs), but also desire to build a highly utilized system, with hopes of leading to higher profitability. Under these considerations, the balance between maximal utilization of a cloud computing system’s (CCS’s) resources is in direct conflict with the cloud user’s interest of high reliability and availability. In other words, the provider is willing to allow a degradation in reliability as long as their profitability continues as, in reality, it is the user, not the provider, that pays the economic consequences of cloud failures. Note that from a user-based, SLA driven perspective, reliability refers to the ability of the cloud to serve the user’s need over some time period and does not refer to simple failures within a CCS that do not hinder user service.

This need to provide a highly reliable, uninterrupted cloud service while effectively utilizing all available resources is highly desired by cloud providers/users and clearly demonstrates a gap in current CCS research, calling for the establishment of efficient methods which can quantitatively evaluate and design CCSs based on the competing needs of users (reliability) and providers (utilization). As such, the goal of this study is the design and evaluation of CCSs considering stochastic failures in the CCS as well as stochastic virtual machine (VM) requests.

In order to achieve this goal, this study makes multiple contributions including 1) Developing a computationally efficient method for evaluating the reliability of CCSs using non-sequential Monte Carlo simulation (MCS) considering stochastic hardware failures and VM requests, 2) Extending this new model in order to design highly reliable and utilized CCSs based on potential workloads, and 3) Discussing the practical implications of the proposed technique. As opposed to most previous work, the proposed method 1) Focuses on simulation-based analysis, 2) Is highly scalable due to the use of MCS, and 3) Uses a newly developed, intuitive system representation.

The remainder of this paper is organized as follows: Section “Related works” reviews background literature that is pertinent to the proposed methodology; Section “Proposed methodologies” presents the the newly proposed application of non-sequential MCS, its formulation for assessing the reliability of a CCS, and its use in a new, iterative algorithm for designing highly reliable and highly utilized CCSs; Section “Experimental results” details the experimental results achieved including CCS test systems designed and evaluated using the proposed methods; Section “Discussion” presents a discussion and comments on using non-sequential MCS as a tool for CCS reliability assessment and the role of this technology in SLAs. Insights gathered during CCS reliability assessments and CCS design are also given in Section “Practical implications”; and, finally, Section “Conclusion” concludes the paper with a summary as well as directions for future work.

Related works

Cloud computing reliability

Many works reference the terms reliability and availability when focused on CCSs. Though, in most cases, the terms refer to increasing system stability through active management [9] or redundancy [10, 11]. While these works begin to lay a strong foundation in this area, they also expose certain gaps in knowledge. Most of these works tend to evaluate either some aspect of QoS or the impact of hardware failures. Many of the initial works focus on the use of Markov chains [1216], as a CCS is effectively a complex network availability problem. Other works focus on conceptual issues [1719], hierarchical graphs [20], the use of grid computing for dynamic scalability in the cloud [21], and priority graphs [22], or the development of performance indices [23].

When considering QoS, one of the largest bodies of work has been completed by Lin and Chang [2429]. These works develop a sequential and systematic methodology based on capacitive flow networks for maintaining the QoS of a CCS with an integrated maintenance budget. The main focus of the model is maintaining acceptable transmission times between clients and providers given a certain budget. The work developed in [30] presents a hierarchical method for evaluating availability of a CCS that is focused on the response time of user requests for resources. The majority of the work deals with VM failure rates, bandwidth bottlenecks, response time, and latency issues. The demonstrated solutions to these issues are the use of their newly developed architecture along with request redirection. A similar, though only conceptual approach, is developed in [31, 32] where a Fault Tolerance Manager (FTM) is developed and inserted between System and Application layers of the CCS. Another approach to this issue is an optimal checkpointing strategy that is used to ensure the availability of a given system [33, 34]. Other methods of approaching fault tolerance from a middleware perspective can be found in [20, 35].

While the previous works have dealt mainly with the modeling of user requests and data transmission, another important aspect of system failure in a CCS is the failure of hardware. The state-of-the-art in this area is embodied in five main works that focus on evaluating data logs from multiple data centers and/or consumer PCs. The evaluation of these logs begins in [36] where hardware failures of multiple data centers are examined to determine explicit rates of failure for different components — namely disks, CPUs, memory, and RAID controllers. The most important finding of this paper is that the largest source of failure in such data centers is disk failure. Intermittent hardware errors are evaluated in [37].

This work continues in [38] where failures in CPU, DRAM, and disks in consumer PCs are evaluated. Special attention is paid to recurring faults as the work suggests that once a PC component fails, it is much more likely to fail again. The paper also examines failures that are not always noticeable to an end-user, such as 1-bit failures in DRAM. A thorough evaluation of failures and reliability at all levels of the CCS is found in [39].

Instead of focusing on internal hardware failures, Gill et al. focus on network failures in data centers [40, 41]. These studies conclude that 1) Data center networks are highly reliable, 2) Switches are highly reliable, 3) Load balancers most often experience faults due to software failures, 4) Network failures typically cause small failures that lose a large number of smaller packets, and 5) Redundancy is useful, but not a perfect solution.

An interesting companion to the study of hardware failures is the large scale performance study performed in [42]. While this study does not explicitly focus on failures or reliability, it does provide a thorough analysis of resource utilization and general workloads in data centers. The work evaluates the utilization of various hardware pieces including CPUs, memory, disks, and entire file systems.

Monte Carlo simulation

MCS is a stochastic simulation tool which is often used to evaluate complex systems as it remains tractable regardless of dimensionality. The MCS algorithm comes in two varieties: non-sequential and sequential. Sequential MCS is typically used to evaluate complex systems that require some aspect of time dependence. Because of this, this variant of the algorithm requires more computational overhead and takes longer to converge. Non-sequential-MCS (referred to as MCS for the remainder of this study) exhibits a higher computational efficiency than sequential MCS. The downside of the non-sequential MCS algorithm is that the rate at which convergence time typically increases with problem dimensionality or system size. Also note that the rate of convergence for MCS is \(1/\sqrt {N}\) where N is the number of samples drawn. This means that convergence does not depend upon dimensionality, allowing MCS to handle problems with a large state space. While this can become an issue, it is easily handled as the MCS algorithm is highly parallel and, in the case of long running simulation requirements, may be easily parallelized in order to quickly simulate complex systems.

The general non-sequential MCS algorithm used for evaluating a CCS in this study is shown in Fig. 1. As the general operation of the MCS requires the repeated sampling of a state space and the evaluation of those states sampled, all four steps of the MCS algorithm (sampling, classification, calculation, and convergence) are dependent on an efficient representation of individual states. This representation as well as further details regarding the implementation of MCS in this study are detailed in the following section.

Fig. 1
figure 1

General MCS algorithm. The generic algorithm used for evaluating system reliability. Note that the “Classify Sampled State” and “Perform Calculations” steps are modified in any implementation

Proposed methodologies

This section presents a review of the non-sequential MCS algorithm in a formulation applicable to CCS reliability evaluation. While this formulation is focused on evaluating the reliability of a CCS, this same algorithm can be used to 1) Evaluate the reliability of an already existing CCS under various loads (or, potentially in real-time) and 2) Design a CCS with a high level of reliability that is also highly utilized. As such, this section also presents an iterative algorithm for the design of a highly reliable and highly utilized CCS.

Such a simulation-based technique is required because, when hardware resources are considered, it is important to look beyond a simple calculations that determine whether or not enough resources are available. A more complex issue is calculating the amount of resources required in light of the stochastic failure rates of hardware resources in the system as coupled with varying user requests for VMs. In such a case, one must look at the state of the system across multiple “snapshots” of existence in order to ensure that enough resources will be available to handle the workload, even when some portion of hardware fails or general usage increases. Non-sequential MCS allows for such an analysis.

System evaluation using MCS

As described in the previous section, the MCS algorithm is highly dependent on an efficient method for state representation in order to achieve convergence through the iterative process of sampling a state, classifying a state, performing any necessary calculations, and then checking convergence. Each of these algorithmic steps are discussed in the subsections below. As the study is focused on evaluating and designing systems with a high level of reliability (the probability of the system functioning during some time period, t) from a user-based perspective, throughout this work the assumption is maintained that the system is measured and evaluated while in use. In other words, unallocated resources and their failures are not considered.

The following sections describe the state representation used in the MCS algorithms as well as each stage of the MCS algorithm used in this study (sampling, classification, and determining convergence). According to the process defined in Fig. 1, the MCS algorithm will use the state representation to repeatedly sample the state space, classify each sampled state, and then determine convergence based on these details.

State representation

In this study, we consider the modeling of a single server that exists inside of a CCS. Such a server can be represented as an Y-bit bit field, X, where Y is the number of resource types being considered. Using a bit field representation is not a new concept as it is commonly used in a variety of disciplines and problem formulations, but the authors are unaware of any use of this methodology to represent CCSs. In the proposed representation, each bit represents the state of a resource; a “1” denotes an up/functioning state and a “0” a down/failed state. This type of state is depicted in Fig. 2. Furthering this representation, the state of a single server can be distilled to a single bit according to (1) where S is a single state with I resources each represented as X i . This methodology results in an entire CCS may be represented as a binary vector with each bit representing the state of a single server, either failed — 0 — or functioning — 1. Since each server can take on 2 possible states, the entire state space will consist of 2N states, where N is the total number of servers. Again, this provides a highly expandable framework for representing and evaluating very large CCSs (i.e. adding a single bit to the binary CCS vector for each additional server). This state representation scheme is highly advantageous, allowing for a high level of customization and extensibility, leading to an array of variations that should be able to model all available cloud computing service types (i.e. IaaS, SaaS, PaaS, etc.). The only change for considering an additional resource type is appending an extra bit to each server’s binary state string as represented by a binary number. For example, if there was a need to extend this model to include a network interface card (NIC) on each server, the bit representation could simply be extended by a single digit. This could be done for any variety of resources.

Fig. 2
figure 2

MCS state representation. An example showing the states of two, individual servers. The server on the left has failed while the server on the right has not failed

One objection that may be raised to this methodology is the lack of inclusion regarding partially failed, de-rated, or grey states. Such states do play an important role, particularly when considering specific resources. For example, portions of a hard drive may be marked as damaged or unusable and, thus, excluded from total resources available. Though, as the state model is highly malleable, de-rated states may be included through the inclusion of a three-or-more state model where the 0/1 model currently suggested is replaced by a 0/1/2 model where zero represents a completely failed resource, one represents a derated resource, and two represents a fully functioning resource. For the purposes of this research, such an extension is left for future work.

For the simulations performed in this study, servers are considered as consisting of CPU, memory, hard disk drive (HDD), and bandwidth resources or P, M, H, and B respectively. Thus, the state of a single server is represented as a 4-bit, bit field (e.g. a state of 1101 represents a server with CPU, memory, and bandwidth in up states and the HDD in a failed state). This state clearly represents the IaaS model of cloud computing (providing requested infrastructure resources) and is chosen as IaaS is the foundation for other types of services (i.e. SaaS is built upon PaaS which is, in turn, built upon IaaS). Accordingly, this state space representation may be expanded to encompass resources specific to each of these models.

$$ S = \prod\limits_{i=1}^{I}X_{i} $$


In order to effectively sample a state from a given state space, a uniform distribution, u, is used. Since the reliability of a device is exponentially distributed according to its annual failure rate (AFR), each uniformly distributed number is transformed into an exponentially distributed number. Thus, u i is transformed into an exponentially distributed random number, r i , using the well-known inversion method, according to (2). An AFR represents the estimated probability that a device will fail during a full year of use. In this study, all AFR values are derived from the work found in [3642].

The binary state string, X, is constructed by generating a series of random values that are compared to each resource’s AFR. Specifically, the value of any given location in the state string will be determined by comparing r i to the AFR of resource i according to (3).

$$\begin{array}{*{20}l} r_{i} &= -\text{ln}{(1 - u_{i})}/{AFR}_{i} \end{array} $$
$$\begin{array}{*{20}l} X_{i} &=\left\{ \begin{array}{ll} 0 & r_{i} \le {AFR}_{i}\\ 1 & \quad \text{otherwise}\\ \end{array} \right. \end{array} $$

Note that AFR is a simplistic measure of system availability in contrast to a more robust measure like forced outage rate (FOR). This is because AFR does not take into account the combination of failure and repair rates that a measure like FOR encompasses. As this is an exploratory study, the authors chose AFR rather than the FOR due to the lack of accurate repair and failure rates for CPUs, HDDs, memory, etc.

State classification

The state classification step of MCS relies on a straightforward comparison of the resources requested and resources available as a measure of system adequacy. Thus, a state will be sampled and the provided resources are compared to those available. For the system to adequately supply the needed resources the relation in (4) must hold for each individual CCS resource as defined below.

$$ Y_{requested} \leq Y_{available} $$

When a CCS supplies more resources than are requested the system will be in a functioning state. In any other case the system will have failed. The mathematics of this method are shown in (5)–(8). Note that this methodology may easily be extended to any number or resources including databases, software packages, etc.

$$ Y_{requested} = \sum\limits_{v=0}^{V} Y_{v} $$
$$ Y_{available} = \sum\limits_{s=0}^{S} Y_{s} $$
$$ Y_{curtailed} =\left\{ \begin{array}{ll} 0 & Y_{requested} \leq Y_{available} \\ 1 & \textnormal{otherwise}\\ \end{array} \right. $$
$$\begin{array}{@{}rcl@{}} S_{x}=\left\{ \begin{array}{ll} 0 & \sum^{Y} Y_{curtailed} > 0\\ 1 & \textnormal{otherwise} \end{array} \right. \end{array} $$

It should be noted that this is an approximation of a real-world scenario. In reality, the assignment and usage of resources is more accurately calculated using a bin packing formulation — an extension that is currently slated for future work.

Determining convergence

In order to evaluate system level performance using MCS, some measure must be calculated in order to determine convergence of the algorithm. As the goal of this study is the evaluation of reliability and utilization, the metric for convergence is defined as R. R is defined as the probability that a CCS will be encountered in a functional state and is defined in (9) and (10) as the ratio of failed states sampled to total states sampled (K). While R is the metric of interest in this study, convergence is determined by the metric F, or the probability that the CCS will be found in a failed state. In order to determine convergence, variance (σ 2) and standard deviation (σ) of the F value are calculated as defined in (11)–(12). Note that it is well known that MCS converges at a rate of \(1/\sqrt {N}\) and that a more detailed derivation of (9)-(12) for MCS can be found in [43].

$$\begin{array}{*{20}l} F &= \frac{1}{K}\sum\limits_{x=1}^{K} S_{x} \end{array} $$
$$\begin{array}{*{20}l} R &= 1 - F = 1 - \frac{1}{K}\sum\limits_{x=1}^{K} S_{x} \end{array} $$
$$\begin{array}{*{20}l} \sigma^{2}(F) &= \frac{1}{K}(F - F^{2}) \end{array} $$
$$\begin{array}{*{20}l} \sigma(F) &= \frac{\sqrt{V(F)}}{F} \end{array} $$

Convergence criteria

The main driver behind the convergence of the MCS algorithm is the sampling of failure states. Accordingly, highly reliable CCS systems will exhibit few such states and will take longer to converge than a system with a state space containing an abundance of failure states. The sampling of failed states drives σ(R) towards 0, to provide an accurate estimate of R. In this study, there are two rules for determining whether the non-sequential MCS algorithm has converged:

$$ (iterations > 10\; \text{and}\; \sigma(R) < 0.080) $$


$$ (iterations > 20,000\; \text{and} \;R > 0.999999). $$

The first convergence criterion provides early termination for simulations that have an extremely low R after the first 10 samples (Generally, a highly unreliable system). The second convergence criterion keeps highly reliable (R>0.999999) CCSs from running for long periods of time due to the very sparse distribution of failed states in the state space.

System design using MCS

While the proposed implementation of MCS is focused on evaluating the reliability of complex CCSs, the same algorithm also has applications in designing highly reliable, highly utilized CCSs. In this study an iterative algorithm for designing such a system (and, potentially expanding that system) is developed. Though this algorithm is used in this study for the design of test systems (i.e. model systems that are used for testing the proposed MCS algorithm), the algorithm may also be used for the planning and design of highly reliable, highly utilized CCSs under predicted loads. The algorithm itself relies on the prospect of increasing the amount of available resources that currently cause resource inadequacies. The additional resources yield simultaneous increases in CCS reliability as well as overall resource utilization. The novel algorithmic process of enhancing CCS reliability and resource utilization when the addition of resources is possible is summarized in Algorithm 1 where R desired and U T I L desired are the desired level of system reliability and system utilization, R actual and U T I L actual are the measured levels of system reliability and utilization, and V M count is the number of VMs currently allocated. While this algorithm may appear to be deterministic, the MCS algorithm embedded inside of the algorithm is stochastic and considers the probabilistic failure of available system resources. This means that the amount of resources available are not increased simply to accommodate the amount of resources requested (which is a simple calculation). Instead, the algorithm increases the amount of resources available in order to handle resource requests while also considering stochastic failures of system resources, thus solving a much more difficult problem and leading to a more robust system design.

If the addition of resources to the CCS is infeasible, one way to control the reliability is to perturb the number of VM allocations as opposed to adjusting the amount of resources. The optimal number of maximum VM allocations to satisfy a pre-specified reliability threshold can be easily calculated by repeatedly applying the non-sequential MCS algorithm. After each MCS iteration, the number of VMs allocated is adjusted up or down if the reliability is higher or lower than the threshold respectively. Conversely, if the reliability is at a desired level and resources cannot be added to improve resource utilization (and the number of maximum VM allocations is satisfactory), excess, under-utilized resources can be removed from the CCS and re-purposed.

Experimental results

This section will present the implementation details of the simulation software as well as an overview of test system design and the different VM allocation schemes that are used. Actual results from CCS reliability simulations are also introduced and analyzed.

Implementation notes

The simulation software is implemented in Java 7 (using IntelliJ IDEA 12) and is run on a Dell Inspiron E6430 with a 64-bit version of Windows 7, an Intel Core i7 @ 2.4 GHz, and 8GB RAM. All of the results are stored in a MySQL database for future analysis.

In each simulation, a CCS is abstracted to consist of a finite pool of available and requested resources. The available resources correspond to the servers that compose a cloud. The requested resources consist of the VMs allocated on the cloud. The specific resources focused on within this study include CPUs, memory, hard disk drives (HDD), and bandwidth. Each simulation resource is tied to a pre-specified AFR. The specific AFRs used in this study are shown in Table 1. The AFRs used were gathered and estimated from practical and theoretical research in the current literature that analyzed hardware failures of CCSs [12, 36, 38, 4042].

Table 1 Annual Failure Rates (AFR) used in the simulations

In order to provide for composition of more complex CCSs as well as reusable resource definitions, the software is built around a hierarchy of components. The most basic component is a single server consisting of a finite pool of CPU, memory, HDD, and bandwidth resources. From here, clusters are constructed as groups of servers and clouds are built from a collection of clusters. Likewise, the requested resources are constructed from a hierarchical abstraction. The most basic unit is a VM consisting of a finite pool of CPU, memory, HDD, and bandwidth resources. Individual VMs are combined into groups to represent the total requested resources. In conjunction with grouping individual VMs together, a probability distribution is assigned in accordance with how each type of VM in that group is to be allocated.

The simulations are setup by specifying a particular set of available resources in the form of a cloud, and a set of requested resources in the form of a VM grouping. The non-sequential Monte-Carlo algorithm is run to convergence for each simulation. It is important to note that individual component failures are not tracked by the MCS algorithm. In reality, a specific component that fails once has been shown to fail more often than its peers. This simplification of the state space allows the non-sequential MCS algorithm to converge more efficiently than one that keeps track of this additional information. Thus, the AFRs used in each simulation apply to all resources, and are never modified based on past inadequacies.

There were two types of simulations performed over the course of this study which differ in the way VMs are allocated. The first scheme allocates VMs in a static manner, in which all trials of a single simulation have identical VM allocations. This scheme results in a tight bound on the variance of the reliability of repeated simulations due to the only stochastic behavior arising within the MCS algorithm. The process for performing a static allocation is shown in Algorithm 2 where V M instances refers to the current set of VMs being allocated on the CCS, V M instance is a single VM instance, P R O B i is the probability of a single type of V M instance occurring, and i refers to a singular VM type.

The second allocation scheme adds a second source of stochastic behavior to the reliability simulation. In this scheme, the allocation of VMs is based on a user specified probability distribution over a discrete set of VM types. Multiple simulations set up identically under this arrangement will have varying VM allocations. This scheme provides a view of highly dynamic VM allocation policies, and provides insight on how to better control the overall reliability in these rapidly changing environments. Algorithm 3 details the process of dynamic VM allocation.

Test systems

Due to a lack of standardized test CCSs within literature, the authors needed to develop test-bed systems for simulation. In order to accomplish this goal, this study began with the small CCS depicted in Table 2. The table depicts the total available server resources as well as the resources consumed by the allocation of one VM. This CCS formulation provided insights into 1) What caused different types of hardware failures, 2) The ways in which convergence of the MCS is reached (from above or below), and 3) The number of iterations required for convergence. Based on this initial system, further and more complex systems were designed in order to test the proposed methodology.

Table 2 Small test-bed CCS used in initial simulations

Static VM allocation

An overview of a test-bed simulation with an allocation of 98 VMs is shown in Table 3 and the reliability averaged over 20 trials is 0.98±0.0025. The results from a typical trial are shown in Figs. 3 and 4. It is evident that the HDD resource is the cause of all 153 failures during the simulation. This is due to the high utilization of the HDD resource (98 %) and comparatively low utilization of all other resources (49 % for CPU and memory and 20 % utilization for bandwidth). This shows that the cloud provider could serve many more VMs if they were to add more HDD to this particular cloud. The extra HDD resource would allow for more VM’s to be allocated on the cloud and in effect allow for a much larger utilization percentage of the other resources.

Table 3 Test-Bed CCS - 98 VM’s
Fig. 3
figure 3

Pattern of reliability convergence — initial simulation. Pattern of reliability convergence for the initial test-bed CCS simulation as defined in Table 3

Fig. 4
figure 4

Component failures by resource type — initial simulation. Component failures by resource type for the initial test-bed CCS simulation as defined in Table 3

As such, another simulation is performed with an extra 10,000 GB of HDD resources to the test-bed CCS (Table 4). The results of this simulation with an allocation of 194 VMs is shown in Figs. 5 and 6. The overview table shows that the utilization of the CPU, memory, and bandwidth resources have almost doubled while the HDD utilization has been decreased slightly. The resulting reliability averaged over 20 trials is 0.9960±0.0003. The additional HDD resources have yielded a substantial increase in total cloud utilization while simultaneously increasing the reliability by around 2.22 %. Consequently, the cloud provider was able to supply an additional 96 VMs to consumers with a much higher reliability than in the previous example. Also noteworthy is that the main resource inadequacy is still HDD. This behavior is due to the significantly higher AFR of HDD versus the other resources. The ability to quickly modify CCS resources in conjunction with the efficiency of the MCS algorithm’s convergence allows for quick and easy design of highly reliable and highly utilized CCSs.

Table 4 Test-bed CCS (with extra HDD) - 194 VM’s
Fig. 5
figure 5

Pattern of reliability convergence — initial simulation plus 10,000 GB HDD. Pattern of reliability convergence for the initial test-bed CCS simulation with 10,000 GB extra HDD resource as defined in Table 4

Fig. 6
figure 6

Component failures — initial simulation plus 10,000 GB HDD. Component failures by resource type for initial test-bed CCS simulation with 10,000 GB extra HDD resource as defined in Table 4

The design of highly reliable and highly utilized clouds strikes a balance between resource utilization that is high enough to use a majority of cloud resources yet is safely below the threshold in which many concurrent failures are likely. Since the reliability of a resource is exponentially distributed in accordance with its AFR, resources with high AFRs must be carefully considered, especially at high utilization percentages. In the previous examples the HDD resource was highly utilized, yet it also has an AFR much higher than any of the other resources considered. This over-utilization of a failure prone resource provides an opportunity to greatly improve CCS reliability. Using the iterative algorithm from section 4.3, the test-bed CCS from Table 4 is optimized to obtain a much higher reliability by allocating only 190 VM’s (rather than the 194 that were previously requested). The resulting R value averaged over 20 trials is 0.999970±0.000031, which is around a 0.4 % improvement. All resource inadequacies are due to HDD again, reinforcing the detrimental effects of high AFRs on a CCS’s reliability. More iterations of Algorithm 1 can be performed in order to further increase CCS reliability, yet resource utilization will be reduced.

Subsequently, more complex CCSs are designed with the insights gained from the test-bed system in mind. After qualifying the impacts of various resource allocations, the authors simulate a real world virtual CCS run by the Extreme Science and Engineering Discovery Environment (XSEDE) partnership [44]. The XSEDE partnership is composed of numerous United States universities. XSEDE is an advanced, powerful, and robust collection of integrated advanced digital resources and services that supports a CCS composed of 16 supercomputers as well as high-end data visualization and data analysis resources. The hardware resources provided by the XSEDE CCS are depicted in Table 5.

Table 5 XSEDE Cloud Resources (Est. Bandwidth)

In order to provide many unique, and realistic VM instances beyond the one in the initial test-bed system, the authors chose to simulate a large subset of the available Amazon EC2 VMs [45]. A listing of the Amazon EC2 VM instances that were used in simulations is shown in Table 6. Using VMs mirrored after the actual Amazon EC2 VMs allows for the exploration of complex CCSs with much more complex allocation configurations, especially when the distribution of VMs is assigned in accordance with a probability distribution.

Table 6 Amazon EC2 VM instances (Est. Bandwidth)

Simulation of the XSEDE CCS

The XSEDE cloud which has many more resources available than the original test-bed system, proves much more difficult in balancing high reliability with high utilization. Initially, allocation of each type of Amazon EC2 VM instance is varied based on the amount of resources provided. Achieving high CCS reliability is quite straightforward in this manner. Yet, finding a balance of EC2 instances that also yielded a high utilization of each resource is highly difficult. This makes sense as the XSEDE cloud is primarily aimed at scientific computing and data visualization which requires allocations that are quite different than the normal EC2 instances. For example, an M1 Small or M1 Medium instance would rarely be allocated on the XSEDE CCS as it would be insufficient for performing large scale scientific calculations. A more likely scenario would be the allocation of VMs which utilize much higher quantities of each resource, such as the Cluster Compute Eight Extra Large EC2 instance. Yet, even when allocation is performed using only the most intensive EC2 VMs there is still an overreliance on CPU and HDD. The EC2 VMs also have bandwidth requests that are minimal compared to the jobs that are most likely performed on the XSEDE supercomputers.

Many configurations can be achieved using all of the EC2 instances which provide high reliability and high resource utilization of 2 out of the 4 resources (almost exclusively CPU and HDD). Bandwidth is always underutilized at around 7–10 %. The main conclusion that can be drawn is that in order to effectively serve VM allocations likened to the EC2 instances, the XSEDE CCS would greatly benefit from additional CPU and HDD resources. The addition of these resources would allow the XSEDE CCS to serve many more VMs under the conditions of this study at an even higher reliability level. Another important observation is that VM instances that are light on resources are very good at increasing utilization to a desired level without sacrificing reliability. This behavior is very similar to the way in which a bucket can be filled to the brim with coarse rocks yet there is always room to add in finer particles to fill in the empty voids.

Results of a simulation performed with the XSEDE CCS are shown in Figs. 7 and 8 with the VM allocation percentages shown in Fig. 9. There are a total of 13,000 VM instances allocated yielding CPU, memory, HDD, and bandwidth utilizations of 0.95, 0.25, 0.98, and 0.08 respectively. Of note is the 5,015,055 sampled states that are required for convergence compared with the simulation displayed in Figs. 5 and 6 in which only 41,082 samples are needed. This increase in sampling illustrates the importance of developing more sophisticated failure-state sampling schemes to speed up MCS convergence for large CCS systems.

Fig. 7
figure 7

Pattern of reliability convergence — XSEDE CCS. Pattern of reliability convergence for the XSEDE CCS (Table 5) simulation using the VM allocation shown in Fig. 9

Fig. 8
figure 8

Component failures by resource type — XSEDE CCS. Component failures by resource type for the XSEDE CCS (Table 5) simulation using the VM allocation shown in Fig. 9

Fig. 9
figure 9

Distribution of EC2 VM instances (Table 6) used in the XSEDE CCS simulation

Probability based VM allocation

Simulations are also performed in which multiple, different VM instances are allocated on the CCS in correspondence with a specified probability distribution. This enables the evaluation of a more realistic set of simulations that showcase how allocation variability affects the overall reliability of a CCS. The most important observation from these simulations is that the reliability of a CCS can vary wildly when a fixed number of VMs are allocated using a specified probability distribution. In the event that a multitude of resource intensive VMs are allocated on the cloud the CCS would be very likely to fail. Conversely, if a majority of the VM allocations are very light on resource requirements the CCS would exhibit 100 % reliability.

For example, a simulation is performed using three of the Amazon EC2 VM instances: M1 Small, M1 Medium, and M1 Large with respective allocation probabilities of 50 %, 30 %, and 20 %, sampled from a uniform distribution. The XSEDE cloud is used as the available resources (Fig. 5) and 31,050 VMs are allocated using the specified distribution. The CCS’s reliability varies from 0.0 to 1.0 over the course of 500 simulations. The average R value is 0.89±0.1912. Fig. 10 shows a histogram of the R values for the 500 simulations with a bin size of 0.01. Although the resulting distribution of CCS reliability is heavily skewed to the highly reliable side, it is very important to note that there are a total of 9 simulations that resulted in immediate, total failures (R value of 0.0). A specific allocation that results in a R of 1.0 was characterized by allocation percentages of 50.8 %, 29.7 %, and 19.4 % for the M1 Small, M1 Medium, and M1 Large VMs. Likewise, an allocation that results in a R of 0.0 consisted of 49.4 %, 30.2 %, and 20.4 % of M1 Small, M1 Medium, and M1 Large VMs. These numbers illustrate how a slight difference in VM allocation can result in a large change in the reliability of a CCS.

Fig. 10
figure 10

Distribution of R values from 500 probability based VM allocation simulations

For comparison, 500 simulations are performed using the exact setup described above except that the VM types are statically allocated. Hence, M1 Small, M1 Medium, and M1 Large occupy 50 %, 30 %, and 20 % of the 31,050 VM allocations respectively for all 500 simulations. This simulation results in a R value of 0.9917±0.0006. The minimum and maximum R values are 0.989304 and 0.993757, respectively. These simulations illustrate that even a simple CCS under a dynamic allocation policy can exhibit highly deceptive reliability characteristics when too few simulations are performed.

This highly erratic reliability behavior is a direct result of the combined use of probabilistic methods of VM allocation and the design of a test system that is highly utilized. In any highly utilized system, a slight variance in workload can easily move a system from highly utilized and stable to over-utilized and unstable. This also showcases the value of the novel application of non-sequential MCS in order to efficiently simulate a CCS numerous times. This behavior becomes even more variable when more than three VM types are used in the probability distribution. The ability to assess the distribution of reliability across a broad range of allocation schemes greatly aids in designing a CCS that maintains high reliability. Without an abundance of simulations, the design of highly reliable CCSs is a futile exercise within such a dynamic environment.


A recurring observation throughout the empirical simulations is the importance of matching VM allocations to hardware that is specifically suited to the type of job at hand. For instance, naively allocating 116,000 small test-bed VM’s on the XSEDE cloud results in a reliability of 98.7 % due to shortage of hard disk resources. In addition, this setup does not efficiently utilize the available CPU (68 %), memory (34 %), or bandwidth (5 %) resources.

Yet, when a hand crafted group of VM’s that are much more CPU and memory intensive are allocated on the XSEDE cloud the total pool of resources are much more efficiently utilized; all while retaining a very high degree of reliability (≥99.9999 %). It is quickly evident that groups of VMs can be engineered to cause any particular type of resource inadequacy. Furthermore, it is very likely that users requesting resources that do not require a large amount of storage space would typically see failures in other components — CPUs, RAM, etc. Yet, this same functionality can be leveraged to craft probability distributions over VM instances that allow for highly reliable clouds with each individual resource being highly utilized. By intelligently allocating the proper types of VM instances, overall cloud reliability can be controlled with a fine degree of precision while efficiently using the available resource pool. In fact, if the probability distribution over VM instances and AFRs are known, limits on CCS reliability can be readily established via simulation.

For instance, Fig. 11 depicts repeated simulations of the XSEDE CCS with varying numbers of VMs allocated. The VM instances used in this simulation were the EC2 instances: M1 Small, M1 Medium, and M1 Large, at distribution percentages of 20 %, 40 %, and 40 %, respectively. The figure shows that the XSEDE CCS can adequately supply resources for 21,567 VMs, while retaining a reliabilty greater than 99.9104 %. If the number of VM allocations is decreased to 21,447, the minimum reliability over 10 trials increases to 99.9992 %. Of course, these results are highly dependent on the accuracy of the AFRs specified at simulation time, as well as the number of trials performed. This method can be extended to establish accurate CCS reliability curves that illustrate CCS reliability characteristics across a wide range of VM allocations.

Fig. 11
figure 11

Minimum CCS reliability based on number of VM Allocations (10 trials)


It is important to note that this methodology considers a worst case scenario in which all VM allocations consume full resources at all times. In reality, a server hypervisor manages the resources requested by each VM, allowing more VM guests to be allocated without detrimental impacts on overall cloud reliability. For instance, one physical CPU core may be mapped to four virtual CPU cores and each guest VM will wait for its share of virtual cores to be available from the hypervisor to run their compute tasks. Similarly, a storage hypervisor can be used to manage virtualized storage resources to increase utilization rates of disk while maintaining high reliability.

Further, this study only considers the availability of the servers hosting the VMs and does not consider other subsystems like external storage, storage area network (SAN), or failures in other systems such as software bugs or human error. There are also other features of the cloud that cannot be modeled by this approach which affect the overall reliability such as live migration of VMs, or specific CCS network topologies such as DCell which have nodes with varying importance to overall CCS reliability. Live migration of VMs can increase the reliability of the cloud by allowing VMs to be seamlessly transferred between servers for load balancing or server repair. The CCS network topology can have large consequences on overall reliability as server centric architectures like DCell have relay nodes that are more important than individual compute nodes for CCS reliability. Further, oversubscription has further implications that may also impact reliability.

Practical implications

The importance of cloud availability and reliability extend beyond academic interest as they also have monetary consequences when considering cloud SLAs. For example, the Amazon EC2 SLA states that Amazon will use commercially reasonable efforts to make Amazon EC2 and Amazon ABS each available with a MUP of at least 99.95 % [46]. Amazon calculates MUP by subtracting the percentage of minutes during the month in which the Amazon service is in a state of “Region Unavailable.” This leaves a mere 21.6 minutes of downtime that Amazon is allotted to meet a MUP of 99.95 % in a 30 day month. In the event that Amazon cannot meet the terms of this SLA, a credit is issued to the consumer in accordance with Table 7. Rackspace, on the other hand, guarantees that their infrastructure will be available 100 % of the time and will issue account credits up to the full monthly fee for affected servers based on the number of hours the server is down [47].

Table 7 SLA service commitments and credits

The cloud resource providers are not the only parties that experience detrimental economic effects from CCS downtime. CCS resource consumers lose money each minute that their website or IT infrastructure is down. According to InformationWeek, IT downtime costs $ 26.5 billion in lost revenue per year [48]. Thus, there is a mutual interest in making CCS availability and resilience approach 100 %. This is in addition to other cloud computing consequences that could arise including data loss and data security. In order to retain its customer base, CCS providers must be highly aware of the consequences of downtime as well as pro-actively pursuing increased reliability by continually re-evaluating their infrastructure to provide highly available and reliable service. This is especially important in light of the sheer number of CCS failures and issues showcased on the IWGCR website [1].

In order to uphold such stringent uptime requirements efficient, effective ways of evaluating and improving cloud reliability and availability are extremely important. In fact, when dealing with such high MUPs, availability, and lost revenue requirements, every minute of uptime is crucial. The methods developed in this study for the design and evaluation of CCSs provides one set of tools for efficiently pursuing this goal. CCS providers can leverage the proposed algorithms to assess their infrastructure from a vantage point of both reliability and utilization. Non-sequential MCS and its derivatives may also be used to validate and assess their SLAs in order to ensure that they are effortlessly meeting the required MUPs. The resource expansion and planning algorithms can be iteratively applied to maximize revenue and minimize costs associated with providing CCS resources to consumers. The decision to add resources to a CCS no longer would be necessitated by a failure, but could be justified by quantitative metrics provided via the proposed methods. These methods can also save time by simulating new CCS designs prior to actually building physical systems to ensure that the resources will be sufficient for the expected VM allocation load.


This study has presented and analyzed a novel approach to assessing the reliability of CCSs using non-sequential MCS. It was shown that non-sequential MCS provides an efficient and flexible way to determine the reliability of a CCS based on a set of discrete resources. A novel algorithm for CCS expansion planning was also introduced which facilitates the design of highly reliable and highly utilized CCSs. Finally, new test-bed CCS systems were developed that can be used for future CCS reliability and availability analyses. Based on the insights garnered during this study, future work may include the following:

  • An extension of the CCS expansion planning algorithm developed in this study to incorporate economic factors to simultaneously maximize CCS reliability and the cloud providers return on investment (ROI);

  • As this study considered a CCS to have only CPU, memory, HDD, and bandwidth resources, a future extension may include graphics processing units (GPUs), databases, software packages, etc. This extension would provide for highly realistic simulations that take into account many more variables than were considered in this preliminary study.

  • An extension of the proposed algorithm to consider partially failed or derated states. For instance, a multiple core CPU may still function at a reduced level if only a subset of the available cores fail;

  • Evaluating implications of server and storage hypervisors on the reliability of a CCS instead of using a simple resources requested vs. available scheme; and

  • This study has made an important approximation — calculating resource assignment and usage in an additive manner. In the future, this will be improved to formulate the resource assignments via a bin packing problem formulation for more realistic results.


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Correspondence to Robert Green.

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The authors declare that they have no competing interests.

Authors’ contributions

BS developed the simulation software, carried out the simulations, analyzed results, and drafted the manuscript. JR aided in simulation analysis and helped to draft the manuscript. RG participated in the conception, design, and implementation of the study and helped to draft the manuscript. VD helped to conceive of and plan the study while also contributing to the manuscript. MA conceived of the study and helped draft the manuscript. All authors read and approved the final manuscript.

Authors’ information

BS received his B.S. in Computer Science and Engineering, B.S. in Electrical Engineering, and M.S. in Computer Science and Engineering from the University of Toledo (UT) in Toledo, OH in 2010 and 2013, respectively. His research interests include the application of computer science to cloud computing, machine learning, artificial intelligence, and signal processing. In 2009, he received the UT Electrical Engineering Student of the Year Award.

JR received his B.S. degree in Computer Science in 2009 and his M.S. degree in Computer Science in 2011 from Bowling Green State University (BGSU). He was awarded his Ph.D. from the University of Toledo in 2014, where his research focused on biomedical imaging and virtual reality-based modeling applications. He is now an Assistant Professor of Computer Science at The University of Findlay, where he is performing research in Human-Computer Interaction (HCI) and usability engineering, in addition to his interests in computer vision and virtual reality.

RG received his B.S. in Computer Science from Geneva College in 2005, his M.S. from Bowling Green State University in 2007, and his Ph.D. from the University of Toledo in 2012. He currently serves as an Assistant Professor at Bowling Green State University. His research is driven by a love for code and covers a variety of areas including High Performance Computing, Population-based Metaheuristics, Software Development, and the application of these interests to the evaluation and analysis of complex networks including the power grid and cloud computing systems.

VD received the B.Eng. degree in EEE and the M.Sc. degree in physics from BITS, Pilani, in 1996, and the Ph.D. in electronics from Carleton University, Canada, in 2003. Since 2008, he is an Associate Professor in the EECS Department at the University of Toledo. Dr. Devabhaktuni’s R&D interests include applied electromagnetics, biomedical applications of wireless sensor networks, computer aided design, device modeling, image processing, infrastructure monitoring, neural networks, optimization methods, power theft modeling and education, RF/microwave devices, and virtual reality.

MA received the B.S. degree in Electrical Engineering with honors from AMU Aligarh, India in 1969. He received the M.E. with distinction and Ph.D. degrees from IISc. Bangalore, in 1971 and 1974 respectively. He served as the Graduate Director of the EECS Department from 1996 - 1998 and the Undergraduate Director of the CSE program from 1998 - 2001. He received 2008 EECS Teacher of the year award, 2008 College of Engineering Outstanding Teacher award and 2006 IEEE Engineer of the year award of the Toledo Section of the IEEE.

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Snyder, B., Ringenberg, J., Green, R. et al. Evaluation and design of highly reliable and highly utilized cloud computing systems. J Cloud Comp 4, 11 (2015).

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  • Cloud computing
  • Reliability
  • System design
  • Monte Carlo simulation