Operating System-level Virtualization is a server virtualization technology which virtualizes servers on a operating system (kernel) layer. It can be thought of as partitioning: a single physical server is sliced into multiple small partitions (otherwise called virtual environments (VE), virtual private servers (VPS), jails, guests, zones etc); each such partition looks and feels like a real server, from the point of view of its owner. On Unix systems, this technology can be thought of as an advanced extension of the standard chroot mechanism.

The operating system level architecture has low overhead that helps to maximize efficient use of server resources. Due to a single-kernel approach, this type of virtualization introduces only a negligible overhead and allows running hundreds of virtual private servers on a single physical server. In contrast, approaches such as emulation (like VMware) and paravirtualization (like Xen or UML) can not achieve such level of density, due to overhead of running multiple kernels. From the other side, operating system-level virtualization does not allow running different operating systems (i.e. different kernels), although different libraries, distributions etc. are possible.

Since there is a single OS kernel which maintains all the partitions, isolation and resource management become very important. Without proper isolation security can be compromised, and without proper resource management an application from a partition can abuse resources and thus cause a denial of service for other partitions. Resources controlled and limited can include: CPU time, disk space, I/O bandwidth, network access, and all the other finite resources (like RAM, shared memory, locked pages, number of processes, socket buffers and so on). For example, OpenVZ provides a set of more than 20 finite resources that are accounted and limited on a per-partition basis.

Applications

OS-level virtualization solutions are popularly used for virtual private servers in web hosting, in which customers rent root (administrator) access to a partition preinstalled with network server software of different kinds, mostly web servers. This approach is very popular due to its very low total cost of ownership for an environment that is more customizable and more secure than shared hosting.

Furthermore, it is common to use such environments for the hosting of control panels to manage shared hosting inside the environment. Due to the very infrequent use of common web pages, this technique makes resource usage very effective, with no visible performance losses, and allowing one to have one's simple pages hosted for just a few dollars per month.

Another popular application is server consolidation. One can significantly decrease the number of physical servers by migrating them into virtual environments, thus saving money on hardware, electricity, and decreasing management efforts.

Implementations

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