Benefits and Features
Several advantagesmake virtualization attractive.Most of themare fundamen- tally related to the ability to share the same hardware yet run several different execution environments (that is, different operating systems) concurrently.
One important advantage of virtualization is that the host system is pro- tected from the virtual machines, just as the virtual machines are protected from each other. A virus inside a guest operating system might damage that operating system but is unlikely to affect the host or the other guests. Because each virtual machine is almost completely isolated from all other virtual machines, there are almost no protection problems.
A potential disadvantage of isolation is that it can prevent sharing of resources. Two approaches to providing sharing have been implemented. First, it is possible to share a file-system volume and thus to share files. Second, it is possible to define a network of virtual machines, each of which can send information over the virtual communications network. The network is mod- eled after physical communication networks but is implemented in software. Of course, the VMM is free to allow any number of its guests to use physical resources, such as a physical network connection (with sharing providedby the VMM), in which case the allowed guests could communicate with each other via the physical network.
One feature common to most virtualization implementations is the ability to freeze, or suspend, a running virtual machine. Many operating systems provide that basic feature for processes, but VMMs go one step further and allow copies and snapshots to be made of the guest. The copy can be used to create a new VM or to move a VM from one machine to another with its current state intact. The guest can then resume where it was, as if on its original machine, creating a clone. The snapshot records a point in time, and the guest can be reset to that point if necessary (for example, if a change was made but is no longer wanted). Often, VMMs allow many snapshots to be taken. For example, snapshots might record a guest’s state every day for amonth, making restoration to any of those snapshot states possible. These abilities are used to good advantage in virtual environments.
A virtual machine system is a perfect vehicle for operating-system research and development. Normally, changing an operating system is a difficult task. Operating systems are large and complex programs, and a change in one part may cause obscure bugs to appear in some other part. The power of the operating system makes changing it particularly dangerous. Because the operating system executes in kernel mode, a wrong change in a pointer could cause an error that would destroy the entire file system. Thus, it is necessary to test all changes to the operating system carefully.
Of course, the operating system runs on and controls the entire machine, so the system must be stopped and taken out of use while changes are made and tested. This period is commonly called system-development time. Since it makes the system unavailable to users, system-development time on shared systems is often scheduled late at night or on weekends, when system load is low.
A virtual-machine system can eliminate much of this latter problem. Sys- tem programmers are given their own virtual machine, and system develop- ment is done on the virtual machine instead of on a physical machine. Normal system operation is disrupted only when a completed and tested change is ready to be put into production.
Another advantage of virtual machines for developers is that multiple operating systems can run concurrently on the developer’s workstation. This virtualized workstation allows for rapid porting and testing of programs in varying environments. In addition, multiple versions of a program can run, each in its own isolated operating system,within one system. Similarly, quality- assurance engineers can test their applications in multiple environments with- out buying, powering, and maintaining a computer for each environment.
A major advantage of virtual machines in production data-center use is system consolidation, which involves taking two or more separate systems and running them in virtual machines on one system. Such physical-to-virtual conversions result in resource optimization, since many lightly used systems can be combined to create one more heavily used system.
Consider, too, thatmanagement tools that are part of the VMMallow system administrators to manage many more systems than they otherwise could. A virtual environment might include 100 physical servers, each running 20 virtual servers. Without virtualization, 2,000 servers would require several system administrators. With virtualization and its tools, the same work can be managed by one or two administrators. One of the tools thatmake this possible is templating, in which one standard virtual machine image, including an installed and configured guest operating system and applications, is saved and used as a source for multiple running VMs. Other features include managing the patching of all guests, backing up and restoring the guests, andmonitoring their resource use.
Virtualization can improve not only resource utilization but also resource management. Some VMMs include a live migration feature that moves a run- ning guest from one physical server to another without interrupting its opera- tion or active network connections. If a server is overloaded, livemigration can thus free resources on the source host while not disrupting the guest. Similarly, when host hardware must be repaired or upgraded, guests can be migrated to other servers, the evacuated host can be maintained, and then the guests can be migrated back. This operation occurs without downtime and without interruption to users.
Think about the possible effects of virtualization on how applications are deployed. If a system can easily add, remove, and move a virtual machine, then why install applications on that system directly? Instead, the application could be preinstalled on a tuned and customized operating system in a vir- tual machine. This method would offer several benefits for application devel- opers. Application management would become easier, less tuning would be required, and technical support of the application would be more straightfor- ward. System administrators would find the environment easier to manage as well. Installation would be simple, and redeploying the application to another system would be much easier than the usual steps of uninstalling and rein- stalling. For widespread adoption of this methodology to occur, though, the format of virtual machines must be standardized so that any virtual machine will run on any virtualization platform. The “Open Virtual Machine Format” is an attempt to provide such standardization, and it could succeed in unifying virtual machine formats.
Virtualization has laid the foundation for many other advances in com- puter facility implementation, management, and monitoring. Cloud comput- ing, for example, is made possible by virtualization in which resources such as CPU, memory, and I/O are provided as services to customers using Internet technologies. By using APIs, a program can tell a cloud computing facility to create thousands of VMs, all running a specific guest operating system and application, that others can access via the Internet. Many multiuser games, photo-sharing sites, and other web services use this functionality.
In the area of desktop computing, virtualization is enabling desktop and laptop computer users to connect remotely to virtual machines located in remote data centers and access their applications as if they were local. This practice can increase security, because no data are stored on local disks at the user’s site. The cost of the user’s computing resource may also decrease. The user must have networking, CPU, and some memory, but all that these system components need to do is display an image of the guest as its runs remotely (via a protocol such as RDP). Thus, they need not be expensive, high-performance components. Other uses of virtualization are sure to follow as it becomes more prevalent and hardware support continues to improve.