What is a virtual machine (VM)?
A virtual machine is a virtual representation, or emulation, of a physical computer. They are often referred to as a guest while the physical machine they run on is referred to as the host.
Virtualization makes it possible to create multiple virtual machines, each with their own operating system (OS) and applications, on a single physical machine. A VM cannot interact directly with a physical computer. Instead, it needs a lightweight software layer called a hypervisor to coordinate between it and the underlying physical hardware. The hypervisor allocates physical computing resources—such as processors, memory, and storage—to each VM. It keeps each VM separate from others so they don’t interfere with each other.
While this technology can go by many names, including virtual server, virtual server instance (VSI) and virtual private server (VPS), this article will simply refer to them as virtual machines.
The below figure provides an overall view of the components that can support a cloud computing provider in making available its services on a market-oriented basis. More specifically, the model applies to PaaS and IaaS providers that explicitly leverage virtualization technologies to serve customers’ needs.
There are four major components of the architecture:
Users and brokers:
They originate the workload that is managed in the cloud datacenter. Users either require virtual machine instances to which to deploy their systems (IaaS scenario) or deploy applications in the virtual environment made available to them by the provider (PaaS scenario). These service requests are issued by service brokers that act on behalf of users and look for the best deal for them.
SLA resource allocator:
The allocator represents the interface between the datacenter and the cloud service provider and the external world. Its main responsibility is ensuring that service requests are satisfied according to the SLA agreed to with the user. Several components coordinate allocator activities in order to realize this goal:
- Service Request Examiner and Admission Control Module: This module operates in the front-end and filters user and broker requests in order to accept those that are feasible given the current status of the system and the workload that is already processing. Accepted requests are allocated and scheduled for execution. IaaS service providers allocate one or more virtual machine instances and make them available to users. PaaS providers identify a suitable collection of computing nodes to which to deploy the users’ applications.
- Pricing Module: This module is responsible for charging users according to the SLA they signed. Different parameters can be considered in charging users; for instance, the most common case for IaaS providers is to charge according to the characteristics of the virtual machines requested in terms of memory, disk size, computing capacity, and the time they are used. It is very common to calculate the usage in time blocks of one hour, but several other pricing schemes exist. PaaS providers can charge users based on the number of requests served by their application or the usage of internal services made available by the development platform to the application while running.
- Accounting Module: This module maintains the actual information on usage of resources and stores the billing information for each user. These data are made available to the Service Request Examiner and Admission Control module when assessing users’ requests. In addition, they constitute a rich source of information that can be mined to identify usage trends and improve the vendor’s service offering.
- Dispatcher: This component is responsible for the low-level operations that are required to realize admitted service requests. In an IaaS scenario, this module instructs the infrastructure to deploy as many virtual machines as are needed to satisfy a user’s request. In a PaaS scenario, this module activates and deploys the user’s application on a selected set of nodes; deployment can happen either within a virtual machine instance or within an appropriate sandboxed environment.
- Resource Monitor: This component monitors the status of the computing resources, either physical or virtual. IaaS providers mostly focus on keeping track of the availability of VMs and their resource entitlements. PaaS providers monitor the status of the distributed middleware, enabling the elastic execution of applications and loading of each node.
- Service Request Monitor: This component keeps track of the execution progress of service requests. The information collected through the Service Request Monitor is helpful for analyzing system performance and for providing quality feedback about the provider’s capability to satisfy requests. For instance, elements of interest are the number of requests satisfied versus the number of incoming requests, the average processing time of a request, or its time to execution. These data are important sources of information for tuning the system.
The SLA allocator executes the main logic that governs the operations of a single datacenter or a collection of datacenters. Features such as failure management are most likely to be addressed by other software modules, which can either be a separate layer or can be integrated within the SLA resource allocator.
Virtual machines (VMs)
Virtual machines constitute the basic building blocks of a cloud computing infrastructure, especially for IaaS providers. VMs represent the unit of deployment for addressing users’ requests. Infrastructure management software is in charge of keeping operational the computing infrastructure backing the provider’s commercial service offering. As we discussed, VMs play a fundamental role in providing an appropriate hosting environment for users’ applications and, at the same time, isolate application execution from the infrastructure, thus preventing applications from harming the hosting environment. Moreover, VMs are among the most important components influencing the QoS with which a user request is served. VMs can be tuned in terms of their emulated hardware characteristics so that the amount of computing resource of the physical hardware allocated to a user can be finely controlled. PaaS providers do not directly expose VMs to the final user, but they may internally leverage virtualization technology in order to fully and securely utilize their own infrastructure. As previously discussed, PaaS providers often leverage given middleware for executing user applications and might use different QoS parameters to charge application execution rather than the emulated hardware profile.
At the lowest level of the reference architecture resides the physical infrastructure that can comprise one or more datacenters. This is the layer that provides the resources to meet service demands.