Virtual private server
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A virtual private server (VPS, also referred to as Virtual Dedicated Server or VDS) is a method of partitioning a physical server computer into multiple servers such that each has the appearance and capabilities of running on its own dedicated machine. Each virtual server can run its own full-fledged operating system, and each server can be independently rebooted.
The practice of partitioning a single server so that it appears as multiple servers has long been common practice in mainframe computers, but has seen a resurgence lately with the development of virtualization software and technologies for other architectures.
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[edit] Overview
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The physical server boots normally. It then runs a program that boots each virtual server within a virtualization environment (similar to an emulator). The virtual servers have no direct access to hardware and are usually booted from a disk image.
There are two kinds of virtualizations: software based and hardware based. In a software based virtualization environment, the virtual machines share the same kernel and actually require the main node's resources. This kind of virtualization normally has many benefits in a web hosting environment because of quota incrementing and decrementing in real time with no need to restart the node. The main examples are Xen, Virtuozzo, Vserver, and OpenVZ (which is the open source and development version of Parallels Virtuozzo Containers).
In a hardware based virtualization, the virtualization mechanism partitions the real hardware resources. In typical implementations, no burst and/or realtime quota modification is possible; the limits are hard and can only be modified by restarting a virtual machine instance[citation needed]. This kind of environment is potentially more secure in the sense that it is less subject to "Quality of Service crosstalk" between VM instances[citation needed]; on the other hand, its security is typically dependent on the correctness of a larger and more complicated Trusted Computing Base[citation needed]. It is more commonly used in enterprise/commercial deployments[citation needed]. Examples include Microsoft Virtual Server, VMware ESX Server, and Xen.
[edit] Uses
Virtual private servers bridge the gap between shared web hosting services and dedicated hosting services, giving independence from other customers of the VPS service in software terms but at less cost than a physical dedicated server. As a VPS runs its own copy of its operating system, customers have superuser-level access to that operating system instance, and can install almost any software that runs on the OS. Certain software does not run well in a virtualized environment, including firewalls, anti-virus clients, and indeed virtualizers themselves; some VPS providers place further restrictions, but they are generally lax compared to those in shared hosting environments. Due to the number of virtualization clients typically running on a single machine, a VPS generally has limited processor time, RAM, and disk space.
Due to their isolated nature, VPSes have become common sandboxes for possibly-insecure public services or update testing. For example, a single physical server might have two virtual private servers running: one hosting the production-level (live) website, and a second which houses a copy of it. When updates to crucial parts of software need to be made, they can be tested in the second VPS, allowing for detailed testing to be conducted without requiring several physical servers.
Virtual private servers are also sometimes employed as honeypots, allowing a machine to deliberately run software with known security flaws without endangering the rest of the server. Multiple honeypots can be quickly set up via VPSes in this fashion.[citation needed]
[edit] Virtual private server hosting
A growing number of companies offer virtual private server hosting, or virtual dedicated server hosting as an extension for Web hosting services. Some web hosting companies call a Virtual Private Server a Virtual Dedicated Server/Dynamic Dedicated Server or the other way around.
[edit] Managed Hosting
Tools are provided to monitor and control the virtual machine.
[edit] Unmanaged Hosting
Typically only minimal services are provided, such as ssh console and reboot. Unmanaged VPS hosting is generally less expensive than managed VPS hosting.
[edit] Unmetered Hosting
Similar to unmanaged hosting but a fixed bitrate is offered so that it is not possible to exceed a monthly budget.
[edit] Virtualization software
For some of the software packages commonly used to provide platform virtualization, see comparison of platform virtual machines.
