OS-level virtualization
OS-level virtualization is an operating system virtualization paradigm in which the kernel allows the existence of multiple isolated user space instances, including containers, zones, virtual private servers, partitions, virtual environments, virtual kernels, and jails. Such instances may look like real computers from the point of view of programs running in them. A computer program running on an ordinary operating system can see all resources of that computer. Programs running inside a container can only see the container's contents and devices assigned to the container.
On Unix-like operating systems, this feature can be seen as an advanced implementation of the standard chroot mechanism, which changes the apparent root folder for the current running process and its children. In addition to isolation mechanisms, the kernel often provides resource-management features to limit the impact of one container's activities on other containers. Linux containers are all based on the virtualization, isolation, and resource management mechanisms provided by the Linux kernel, notably Linux namespaces and cgroups.
Although the word container most commonly refers to OS-level virtualization, it is sometimes used to refer to fuller virtual machines operating in varying degrees of concert with the host OS, such as Microsoft's Hyper-V containers. For an overview of virtualization since 1960, see Timeline of virtualization technologies.
Operation
On ordinary operating systems for personal computers, a computer program can see all the system's resources. They include:- Hardware capabilities that can be employed, such as the CPU and the network connection
- Data that can be read or written, such as files, folders and network shares
- Connected peripherals it can interact with, such as webcam, printer, scanner, or fax
With operating-system-virtualization, or containerization, it is possible to run programs within containers, to which only parts of these resources are allocated. A program expecting to see the whole computer, once run inside a container, can only see the allocated resources and believes them to be all that is available. Several containers can be created on each operating system, to each of which a subset of the computer's resources is allocated. Each container may contain any number of computer programs. These programs may run concurrently or separately, and may even interact with one another.
Containerization has similarities to application virtualization: In the latter, only one computer program is placed in an isolated container and the isolation applies to file system only.
Uses
Operating-system level virtualization is commonly used in virtual hosting environments, where it is useful for securely allocating finite hardware resources among a large number of mutually-distrusting users. System administrators may also use it for consolidating server hardware by moving services on separate hosts into containers on the one server.Operating-system level virtualization can also be used to run software created for a certain Linux distribution on another distribution, an example is Distrobox.
Other typical scenarios include separating several programs to separate containers for improved security, hardware independence, and added resource management features. The improved security provided by the use of a chroot mechanism, however, is not perfect. Operating-system-level virtualization implementations capable of live migration can also be used for dynamic load balancing of containers between nodes in a cluster.
Overhead
Operating-system level virtualization usually imposes less overhead than full virtualization because programs in OS-level virtual partitions use the operating system's normal system call interface and do not need to be subjected to emulation or be run in an intermediate virtual machine, as is the case with full virtualization and paravirtualization. This form of virtualization also does not require hardware support for efficient performance.Flexibility
Operating-system level virtualization is not as flexible as other virtualization approaches since it cannot host a guest operating system different from the host one, or a different guest kernel. For example, with Linux, different distributions are fine, but other operating systems such as Windows cannot be hosted. Operating systems using variable input systematics are subject to limitations within the virtualized architecture. Adaptation methods including cloud-server relay analytics maintain the OS-level virtual environment within these applications.Solaris partially overcomes the limitation described above with its branded zones feature, which provides the ability to run an environment within a container that emulates an older Solaris 8 or 9 version in a Solaris 10 host. Linux branded zones are also available on x86-based Solaris systems, providing a complete Linux user space and support for the execution of Linux applications; additionally, Solaris provides utilities needed to install Red Hat Enterprise Linux 3.x or CentOS 3.x Linux distributions inside "lx" zones. However, in 2010 Linux branded zones were removed from Solaris; in 2014 they were reintroduced in Illumos, which is the open source Solaris fork, supporting 32-bit Linux kernels.