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 appearanceand 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 partitioninga 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 andtechnologies for other architectures.
Overview
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 tohardware 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 machinesshare 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 anddecrementing in real time with no need to restart the node.The main examples are Virtuozzo, HyperVM, Vserver, and OpenVZ which is the core kernel of both Virtuozzo and HyperVM. In a hardware based virtualization, the virtualization mechanism partitionsthe 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 machineinstance. This kind of environment is potentially more secure in the sense that it is less subject to "Quality of Service crosstalk" between VM instances; on the other hand, its security is typicallydependent on the correctness of a larger and more complicated Trusted Computing Base. It is more commonly used in enterprise/commercial deployments. Examples include Microsoft Virtual Server, VMwareESX Server, and Xen.
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 lesscost 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 softwarethat 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 furtherrestrictions, but they are generally lax compared to those in shared hosting environments.
Due to the number of virtualization clients typically run on a single machine, a VPS generally has limited processor time, RAM, and disk space. Due to their isolated nature, VPSes have becomecommon 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 withoutrequiring 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. Multiplehoneypots can be quickly set up via VPSes in this fashion.
