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OS-level virtualization

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OS-level virtualization is an operating system (OS) virtualization paradigm in which the kernel allows the existence of multiple isolated user space instances, called containers (LXC, Solaris containers, AIX WPARs, HP-UX SRP Containers, Docker, Podman), zones (Solaris containers), virtual private servers (OpenVZ), partitions, virtual environments (VEs), virtual kernels (DragonFly BSD), or jails (FreeBSD jail or chroot jail).[1] 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 (connected devices, files and folders, network shares, CPU power, quantifiable hardware capabilities) of that computer. However, programs running inside of 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.[2]

The term container, while most popularly referring to OS-level virtualization systems, is sometimes ambiguously used to refer to fuller virtual machine environments operating in varying degrees of concert with the host OS, e.g., Microsoft's Hyper-V containers. A more historic overview of virtualization in general since 1960 can be found in the Timeline of virtualization development.



On ordinary operating systems for personal computers, a computer program can see (even though it might not be able to access) 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

The operating system may be able to allow or deny access to such resources based on which program requests them and the user account in the context in which it runs. The operating system may also hide those resources, so that when the computer program enumerates them, they do not appear in the enumeration results. Nevertheless, from a programming point of view, the computer program has interacted with those resources and the operating system has managed an act of interaction.

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.



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.

Other typical scenarios include separating several programs to separate containers for improved security, hardware independence, and added resource management features.[3] The improved security provided by the use of a chroot mechanism, however, is not perfect.[4] Operating-system-level virtualization implementations capable of live migration can also be used for dynamic load balancing of containers between nodes in a cluster.



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 (such as VMware ESXi, QEMU, or Hyper-V) and paravirtualization (such as Xen or User-mode Linux). This form of virtualization also does not require hardware support for efficient performance.



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.[5]

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 (referred to as "lx" 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.[6][7] 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.[8]



Some implementations provide file-level copy-on-write (CoW) mechanisms. (Most commonly, a standard file system is shared between partitions, and those partitions that change the files automatically create their own copies.) This is easier to back up, more space-efficient and simpler to cache than the block-level copy-on-write schemes common on whole-system virtualizers. Whole-system virtualizers, however, can work with non-native file systems and create and roll back snapshots of the entire system state.


Mechanism Operating system License Actively developed since or between Features
File system isolation Copy on write Disk quotas I/O rate limiting Memory limits CPU quotas Network isolation Nested virtualization Partition checkpointing and live migration Root privilege isolation
chroot Most UNIX-like operating systems Varies by operating system 1982 Partial[a] No No No No No No Yes No No
Docker Linux,[10] Windows x64[11] macOS[12] Apache license 2.0 2013 Yes Yes Not directly Yes (since 1.10) Yes Yes Yes Yes Only in experimental mode with CRIU [1] Yes (since 1.10)
(security context)
Linux, Windows Server 2016 GNU GPLv2 2001 Yes Yes Yes Yes[b] Yes Yes Partial[c] ? No Partial[d]
lmctfy Linux Apache license 2.0 2013–2015 Yes Yes Yes Yes[b] Yes Yes Partial[c] ? No Partial[d]
LXC Linux GNU GPLv2 2008 Yes[14] Yes Partial[e] Partial[f] Yes Yes Yes Yes Yes Yes[14]
Singularity Linux BSD Licence 2015[15] Yes[16] Yes Yes No No No No No No Yes[17]
OpenVZ Linux GNU GPLv2 2005 Yes Yes[18] Yes Yes[g] Yes Yes Yes[h] Partial[i] Yes Yes[j]
Virtuozzo Linux, Windows Trialware 2000[22] Yes Yes Yes Yes[k] Yes Yes Yes[h] Partial[l] Yes Yes
Solaris Containers (Zones) illumos (OpenSolaris),
2004 Yes Yes (ZFS) Yes Partial[m] Yes Yes Yes[n][25][26] Partial[o] Partial[p][q] Yes[r]
FreeBSD jail FreeBSD, DragonFly BSD BSD License 2000[28] Yes Yes (ZFS) Yes[s] Yes Yes[29] Yes Yes[30] Yes Partial[31][32] Yes[33]
vkernel DragonFly BSD BSD Licence 2006[34] Yes[35] Yes[35] ? Yes[36] Yes[36] Yes[37] ? ? Yes
sysjail OpenBSD, NetBSD BSD License 2006–2009 Yes No No No No No Yes No No ?
WPARs AIX Commercial proprietary software 2007 Yes No Yes Yes Yes Yes Yes[t] No Yes[39] ?
iCore Virtual Accounts Windows XP Freeware 2008 Yes No Yes No No No No ? No ?
Sandboxie Windows GNU GPLv3 2004 Yes Yes Partial No No No Partial No No Yes
systemd-nspawn Linux GNU LGPLv2.1+ 2010 Yes Yes Yes[40][41] Yes[40][41] Yes[40][41] Yes[40][41] Yes ? ? Yes
Turbo Windows Freemium 2012 Yes No No No No No Yes No No Yes
rkt (rocket) Linux Apache license 2.0 2014[42]–2018 Yes Yes Yes Yes Yes Yes Yes ? ? Yes

Linux containers not listed above include:

See also



  1. ^ Root user can easily escape from chroot. Chroot was never supposed to be used as a security mechanism.[9]
  2. ^ a b Using the CFQ scheduler, there is a separate queue per guest.
  3. ^ a b Networking is based on isolation, not virtualization.
  4. ^ a b A total of 14 user capabilities are considered safe within a container. The rest may cannot be granted to processes within that container without allowing that process to potentially interfere with things outside that container.[13]
  5. ^ Disk quotas per container are possible when using separate partitions for each container with the help of LVM, or when the underlying host filesystem is btrfs, in which case btrfs subvolumes are automatically used.
  6. ^ I/O rate limiting is supported when using Btrfs.
  7. ^ Available since Linux kernel 2.6.18-028stable021. Implementation is based on CFQ disk I/O scheduler, but it is a two-level schema, so I/O priority is not per-process, but rather per-container.[19]
  8. ^ a b Each container can have its own IP addresses, firewall rules, routing tables and so on. Three different networking schemes are possible: route-based, bridge-based, and assigning a real network device (NIC) to a container.
  9. ^ Docker containers can run inside OpenVZ containers.[20]
  10. ^ Each container may have root access without possibly affecting other containers.[21]
  11. ^ Available since version 4.0, January 2008.
  12. ^ Docker containers can run inside Virtuozzo containers.[23]
  13. ^ Yes with illumos[24]
  14. ^ See Solaris network virtualization and resource control for more details.
  15. ^ Only when top level is a KVM zone (illumos) or a kz zone (Oracle).
  16. ^ Starting in Solaris 11.3 Beta, Solaris Kernel Zones may use live migration.
  17. ^ Cold migration (shutdown-move-restart) is implemented.
  18. ^ Non-global zones are restricted so they may not affect other zones via a capability-limiting approach. The global zone may administer the non-global zones.[27]
  19. ^ Check the "allow.quotas" option and the "Jails and file systems" section on the FreeBSD jail man page for details.
  20. ^ Available since TL 02.[38]


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  2. ^ Rami, Rosen. "Namespaces and Cgroups, the basis of Linux Containers" (PDF). Retrieved 18 August 2016.
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  5. ^ Huang, D. (2015). "Experiences in using os-level virtualization for block I/O". Proceedings of the 10th Parallel Data Storage Workshop. pp. 13–18. doi:10.1145/2834976.2834982. ISBN 9781450340083. S2CID 3867190.
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  7. ^ "System administration guide: Oracle Solaris containers-resource nanagement and Oracle Solaris zones, Chapter 31: About branded zones and the Linux branded zone". Oracle Corporation. 2010. Retrieved 2014-09-02.
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  10. ^ "Docker drops LXC as default execution environment". InfoQ.
  11. ^ "Install Docker desktop on Windows | Docker documentation". Docker. 9 February 2023.
  12. ^ "Get started with Docker desktop for Mac". Docker documentation. December 6, 2019.
  13. ^ "Paper - Linux-VServer". linux-vserver.org.
  14. ^ a b Graber, Stéphane (1 January 2014). "LXC 1.0: Security features [6/10]". Retrieved 12 February 2014. LXC now has support for user namespaces. [...] LXC is no longer running as root so even if an attacker manages to escape the container, he'd find himself having the privileges of a regular user on the host.
  15. ^ "Sylabs brings Singularity containers into commercial HPC | Top 500 supercomputer sites". www.top500.org.
  16. ^ "SIF — Containing your containers". www.sylabs.io. 14 March 2018.
  17. ^ Kurtzer, Gregory M.; Sochat, Vanessa; Bauer, Michael W. (May 11, 2017). "Singularity: Scientific containers for mobility of compute". PLOS ONE. 12 (5): e0177459. Bibcode:2017PLoSO..1277459K. doi:10.1371/journal.pone.0177459. PMC 5426675. PMID 28494014.
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  19. ^ "I/O priorities for containers". OpenVZ Virtuozzo Containers Wiki.
  20. ^ "Docker inside CT".
  21. ^ "Container". OpenVZ Virtuozzo Containers Wiki.
  22. ^ "Initial public prerelease of Virtuozzo (named ASPcomplete at that time)".
  23. ^ "Parallels Virtuozzo now provides native support for Docker".
  24. ^ Pijewski, Bill. "Our ZFS I/O throttle".
  25. ^ Network virtualization and resource control (Crossbow) FAQ Archived 2008-06-01 at the Wayback Machine
  26. ^ "Managing network virtualization and network resources in Oracle® Solaris 11.2". docs.oracle.com.
  27. ^ Oracle Solaris 11.1 administration, Oracle Solaris zones, Oracle Solaris 10 zones and resource management E29024.pdf, pp. 356–360. Available within an archive.
  28. ^ "Contain your enthusiasm - Part two: Jails, zones, OpenVZ, and LXC". Jails were first introduced in FreeBSD 4.0 in 2000
  29. ^ "Hierarchical resource limits - FreeBSD Wiki". Wiki.freebsd.org. 2012-10-27. Retrieved 2014-01-15.
  30. ^ "Implementing a clonable network stack in the FreeBSD kernel" (PDF). usenix.org. 2003-06-13.
  31. ^ "VPS for FreeBSD". Retrieved 2016-02-20.
  32. ^ "[Announcement] VPS // OS virtualization // alpha release". 31 August 2012. Retrieved 2016-02-20.
  33. ^ "3.5. Limiting your program's environment". Freebsd.org. Retrieved 2014-01-15.
  34. ^ Matthew Dillon (2006). "sys/vkernel.h". BSD cross reference. DragonFly BSD.
  35. ^ a b "vkd(4) — Virtual kernel disc". DragonFly BSD. treats the disk image as copy-on-write.
  36. ^ a b Sascha Wildner (2007-01-08). "vkernel, vcd, vkd, vke — virtual kernel architecture". DragonFly miscellaneous information manual. DragonFly BSD.
  37. ^ "vkernel, vcd, vkd, vke - virtual kernel architecture". DragonFly On-Line Manual Pages. DragonFly BSD.
  38. ^ "IBM fix pack information for: WPAR network isolation - United States". ibm.com. 21 July 2011.
  39. ^ "Live application mobility in AIX 6.1". www.ibm.com. June 3, 2008.
  40. ^ a b c d "systemd-nspawn". www.freedesktop.org.
  41. ^ a b c d "2.3. Modifying control groups Red Hat Enterprise Linux 7". Red Hat Customer portal.
  42. ^ Polvi, Alex. "CoreOS is building a container runtime, rkt". CoreOS Blog. Archived from the original on 2019-04-01. Retrieved 12 March 2019.
  43. ^ "LXD". linuxcontainers.org. Retrieved 2021-02-11.
  44. ^ Rootless containers with Podman and fuse-overlayfs, CERN workshop, 2019-06-04
  45. ^ "Overview — Charliecloud 0.25 documentation". Retrieved 4 October 2020.
  46. ^ "Home". katacontainers.io.
  47. ^ "Bottlerocket is a Linux-based operating system purpose-built to run containers".