A virtual private server (VPS, also referred to as Virtual Dedicated Server or VDS) is a method of splitting a server. Each virtual server can run its own full-fledged operating system, and each server can be separately rebooted.
The practice of partitioning a single server so that it appears as manifold 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.
Overview
The physical server boots normally. It then runs a program that boots each virtual server within a virtualization environment (comparable to an emulator). The virtual servers have no straight 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 in fact require the main node's resources. This kind of virtualization usually has many benefits in a web hosting environment because of quota incrementing and decrementing in real time with no need of restarting 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 surroundings 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 characteristically dependent on the correctness of a bigger and more complicated Trusted Computing Base[citation needed]. It is more normally used in enterprise/commercial deployments[citation needed]. Examples include Microsoft Virtual Server, Virtuozzo,VMware ESX Server, and Xen.
Uses
Virtual private servers bridge the gap between shared web hosting services and dedicated hosting services, giving sovereignty 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 approximately 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 additional 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 usually has limited processor time, RAM, and disk space.
Due to their isolated nature, VPSs have become common sandboxes for possibly-insecure public services or update testing. For instance, 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 a number of physical servers.
Virtual private servers are also from time to time employed as honeypots, allowing a machine to intentionally run software with known security flaws without endangering the rest of the server. Multiple honeypots can be quickly set up via VPSs in this fashion.[1]
Virtual private server hosting
A growing number of companies offer virtual private server hosting, or virtual dedicated server hosting as an addition 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.
Unmanaged Hosting
The customer is left to monitor and administer their own server.
Unmetered Hosting
Similar to unmanaged hosting but a set bitrate is offered so that it is not possible to exceed a monthly budget.
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