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OS-level virtualization
Operating system paradigm allowing multiple isolated user space instances

OS-level virtualization is an operating system (OS) virtualization paradigm in which the kernel allows the existence of multiple isolated user space instances, including 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), and jails (FreeBSD jail and chroot). 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. 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.

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Operation

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.

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.

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.

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 (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.

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.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.67 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

Storage

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.

Implementations

MechanismOperating systemLicenseActively developed since or betweenFeatures
File system isolationCopy on writeDisk quotasI/O rate limitingMemory limitsCPU quotasNetwork isolationNested virtualizationPartition checkpointing and live migrationRoot privilege isolation
chrootMost UNIX-like operating systemsVaries by operating system1982Partial9NoNoNoNoNoNoYesNoNo
DockerLinux,10 Windows x6411 macOS12Apache License 2.02013YesYesPartial13Yes (since 1.10)YesYesYesYesOnly in experimental mode with CRIU [1]Yes (since 1.10)
Linux-VServer(security context)Linux, Windows Server 2016GNU GPLv22001YesYesYesYes14YesYesPartial15?NoPartial16
lmctfyLinuxApache License 2.02013–2015YesYesYesYes17YesYesPartial18?NoPartial19
LXCLinuxGNU GPLv22008Yes20YesPartial21Partial22YesYesYesYesYesYes23
SingularityLinuxBSD Licence201524Yes25YesYesNoNoNoNoNoNoYes26
OpenVZLinuxGNU GPLv22005YesYes27YesYes28YesYesYes29Partial30YesYes31
VirtuozzoLinux, WindowsTrialware200032YesYesYesYes33YesYesYes34Partial35YesYes
Solaris Containers (Zones)illumos (OpenSolaris),SolarisCDDL,Proprietary2004YesYes (ZFS)YesPartial36YesYesYes373839Partial40Partial4142Yes43
FreeBSD jailFreeBSD, DragonFly BSDBSD License200044YesYes (ZFS)Yes45YesYes46YesYes47YesPartial4849Yes50
vkernelDragonFly BSDBSD Licence200651Yes52Yes53—?Yes54Yes55Yes56??Yes
sysjailOpenBSD, NetBSDBSD License2006–2009YesNoNoNoNoNoYesNoNo?
WPARsAIXCommercial proprietary software2007YesNoYesYesYesYesYes57NoYes58?
iCore Virtual AccountsWindows XPFreeware2008YesNoYesNoNoNoNo?No?
SandboxieWindowsGNU GPLv32004YesYesPartialNoNoNoPartialNoNoYes
systemd-nspawnLinuxGNU LGPLv2.1+2010YesYesYes5960Yes6162Yes6364Yes6566Yes??Yes
TurboWindowsFreemium2012YesNoNoNoNoNoYesNoNoYes
rkt (rocket)LinuxApache License 2.0201467–2018YesYesYesYesYesYesYes??Yes

Linux containers not listed above include:

  • LXD, an alternative wrapper around LXC developed by Canonical68
  • Podman,69 an advanced Kubernetes ready root-less secure drop-in replacement for Docker with support for multiple container image formats, including OCI and Docker images
  • Charliecloud, a set of container tools used on HPC systems70
  • Kata Containers MicroVM Platform71
  • Bottlerocket is a Linux-based open-source operating system that is purpose-built by Amazon Web Services for running containers on virtual machines or bare metal hosts72
  • Azure Linux is an open-source Linux distribution that is purpose-built by Microsoft Azure and similar to Fedora CoreOS

See also

Notes

References

  1. Hogg, Scott (2014-05-26). "Software containers: Used more frequently than most realize". Network World. Network world, Inc. Retrieved 2015-07-09. There are many other OS-level virtualization systems such as: Linux OpenVZ, Linux-VServer, FreeBSD Jails, AIX Workload Partitions (WPARs), HP-UX Containers (SRP), Solaris Containers, among others. https://www.networkworld.com/article/749098/cisco-subnet-software-containers-used-more-frequently-than-most-realize.html ↩

  2. Rami, Rosen. "Namespaces and Cgroups, the basis of Linux Containers" (PDF). Retrieved 18 August 2016. http://www.netdevconf.org/1.1/proceedings/slides/rosen-namespaces-cgroups-lxc.pdf ↩

  3. "Secure Bottlerocket deployments on Amazon EKS with KubeArmor | Containers". aws.amazon.com. 2022-10-20. Retrieved 2023-06-20. https://aws.amazon.com/blogs/containers/secure-bottlerocket-deployments-on-amazon-eks-with-kubearmor/ ↩

  4. Korff, Yanek; Hope, Paco; Potter, Bruce (2005). Mastering FreeBSD and OpenBSD security. O'Reilly Series. O'Reilly Media, Inc. p. 59. ISBN 0596006268. 0596006268 ↩

  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. 9781450340083 ↩

  6. "System administration guide: Oracle Solaris containers-resource management and Oracle Solaris zones, Chapter 16: Introduction to Solaris zones". Oracle Corporation. 2010. Retrieved 2014-09-02. http://docs.oracle.com/cd/E19044-01/sol.containers/817-1592/zones.intro-1/index.html ↩

  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. http://docs.oracle.com/cd/E19044-01/sol.containers/817-1592/gchhy/index.html ↩

  8. Bryan Cantrill (2014-09-28). "The dream is alive! Running Linux containers on an illumos kernel". slideshare.net. Retrieved 2014-10-10. https://www.slideshare.net/bcantrill/illumos-lx ↩

  9. Root user can easily escape from chroot. Chroot was never supposed to be used as a security mechanism.[9] ↩

  10. "Docker drops LXC as default execution environment". InfoQ. http://www.infoq.com/news/2014/03/docker_0_9 ↩

  11. "Install Docker desktop on Windows | Docker documentation". Docker. 9 February 2023. https://docs.docker.com/desktop/install/windows-install/ ↩

  12. "Get started with Docker desktop for Mac". Docker documentation. December 6, 2019. https://docs.docker.com/docker-for-mac/ ↩

  13. For btrfs, overlay2, windowsfilter, and zfs storage drivers. [13] ↩

  14. Using the CFQ scheduler, there is a separate queue per guest. /wiki/CFQ ↩

  15. Networking is based on isolation, not virtualization. ↩

  16. 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.[14] ↩

  17. Using the CFQ scheduler, there is a separate queue per guest. /wiki/CFQ ↩

  18. Networking is based on isolation, not virtualization. ↩

  19. 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.[14] ↩

  20. 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. https://www.stgraber.org/2014/01/01/lxc-1-0-security-features/ ↩

  21. 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. /wiki/Logical_Volume_Manager_(Linux) ↩

  22. I/O rate limiting is supported when using Btrfs. /wiki/Btrfs ↩

  23. 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. https://www.stgraber.org/2014/01/01/lxc-1-0-security-features/ ↩

  24. "Sylabs brings Singularity containers into commercial HPC | Top 500 supercomputer sites". www.top500.org. https://www.top500.org/news/sylabs-brings-singularity-containers-into-commercial-hpc/ ↩

  25. "SIF — Containing your containers". www.sylabs.io. 14 March 2018. https://www.sylabs.io/2018/03/sif-containing-your-containers/ ↩

  26. 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. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5426675 ↩

  27. Bronnikov, Sergey. "Comparison on OpenVZ wiki page". OpenVZ Wiki. OpenVZ. Retrieved 28 December 2018. https://wiki.openvz.org/Comparison ↩

  28. 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.[20] ↩

  29. 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. /wiki/Network_interface_controller ↩

  30. Docker containers can run inside OpenVZ containers.[21] ↩

  31. Each container may have root access without possibly affecting other containers.[22] ↩

  32. "Initial public prerelease of Virtuozzo (named ASPcomplete at that time)". http://www.paul.sladen.org/vserver/aspcomplete/2000-08-25/ve-0.4.2-for-2.4.0-test6.diff.gz ↩

  33. Available since version 4.0, January 2008. ↩

  34. 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. /wiki/Network_interface_controller ↩

  35. Docker containers can run inside Virtuozzo containers.[24] ↩

  36. Yes with illumos[25] ↩

  37. See Solaris network virtualization and resource control for more details. /wiki/Solaris_network_virtualization_and_resource_control ↩

  38. Network virtualization and resource control (Crossbow) FAQ Archived 2008-06-01 at the Wayback Machine http://www.opensolaris.org/os/project/crossbow/faq/ ↩

  39. "Managing network virtualization and network resources in Oracle® Solaris 11.2". docs.oracle.com. https://docs.oracle.com/cd/E36784_01/html/E36813/index.html ↩

  40. Only when top level is a KVM zone (illumos) or a kz zone (Oracle). ↩

  41. Starting in Solaris 11.3 Beta, Solaris Kernel Zones may use live migration. ↩

  42. Cold migration (shutdown-move-restart) is implemented. ↩

  43. 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.[28] ↩

  44. "Contain your enthusiasm - Part two: Jails, zones, OpenVZ, and LXC". Jails were first introduced in FreeBSD 4.0 in 2000 http://www.cybera.ca/news-and-events/tech-radar/contain-your-enthusiasm-part-two-jails-zones-openvz-and-lxc/ ↩

  45. Check the "allow.quotas" option and the "Jails and file systems" section on the FreeBSD jail man page for details. http://www.freebsd.org/cgi/man.cgi?query%3Djail&sektion%3D8 ↩

  46. "Hierarchical resource limits - FreeBSD Wiki". Wiki.freebsd.org. 2012-10-27. Retrieved 2014-01-15. http://wiki.freebsd.org/Hierarchical_Resource_Limits ↩

  47. "Implementing a clonable network stack in the FreeBSD kernel" (PDF). usenix.org. 2003-06-13. http://static.usenix.org/publications/library/proceedings/usenix03/tech/freenix03/full_papers/zec/zec.pdf ↩

  48. "VPS for FreeBSD". Retrieved 2016-02-20. http://www.7he.at/freebsd/vps/ ↩

  49. "[Announcement] VPS // OS virtualization // alpha release". 31 August 2012. Retrieved 2016-02-20. https://forums.freebsd.org/threads/34284/ ↩

  50. "3.5. Limiting your program's environment". Freebsd.org. Retrieved 2014-01-15. http://www.freebsd.org/doc/en/books/developers-handbook/secure-chroot.html ↩

  51. Matthew Dillon (2006). "sys/vkernel.h". BSD cross reference. DragonFly BSD. /wiki/Matthew_Dillon ↩

  52. "vkd(4) — Virtual kernel disc". DragonFly BSD. treats the disk image as copy-on-write. http://mdoc.su/d/vkd.4 ↩

  53. "vkd(4) — Virtual kernel disc". DragonFly BSD. treats the disk image as copy-on-write. http://mdoc.su/d/vkd.4 ↩

  54. Sascha Wildner (2007-01-08). "vkernel, vcd, vkd, vke — virtual kernel architecture". DragonFly miscellaneous information manual. DragonFly BSD. "vkernel, vcd, vkd, vke - virtual kernel architecture". DragonFly miscellaneous information manual. http://bxr.su/d/share/man/man7/vkernel.7 ↩

  55. Sascha Wildner (2007-01-08). "vkernel, vcd, vkd, vke — virtual kernel architecture". DragonFly miscellaneous information manual. DragonFly BSD. "vkernel, vcd, vkd, vke - virtual kernel architecture". DragonFly miscellaneous information manual. http://bxr.su/d/share/man/man7/vkernel.7 ↩

  56. "vkernel, vcd, vkd, vke - virtual kernel architecture". DragonFly On-Line Manual Pages. DragonFly BSD. http://mdoc.su/d/vke.4 ↩

  57. Available since TL 02.[39] ↩

  58. "Live application mobility in AIX 6.1". www.ibm.com. June 3, 2008. http://www.ibm.com/developerworks/aix/library/au-aix61mobility/index.html ↩

  59. "systemd-nspawn". www.freedesktop.org. https://www.freedesktop.org/software/systemd/man/systemd-nspawn.html#--property= ↩

  60. "2.3. Modifying control groups Red Hat Enterprise Linux 7". Red Hat Customer portal. https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/7/html/resource_management_guide/sec-modifying_control_groups ↩

  61. "systemd-nspawn". www.freedesktop.org. https://www.freedesktop.org/software/systemd/man/systemd-nspawn.html#--property= ↩

  62. "2.3. Modifying control groups Red Hat Enterprise Linux 7". Red Hat Customer portal. https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/7/html/resource_management_guide/sec-modifying_control_groups ↩

  63. "systemd-nspawn". www.freedesktop.org. https://www.freedesktop.org/software/systemd/man/systemd-nspawn.html#--property= ↩

  64. "2.3. Modifying control groups Red Hat Enterprise Linux 7". Red Hat Customer portal. https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/7/html/resource_management_guide/sec-modifying_control_groups ↩

  65. "systemd-nspawn". www.freedesktop.org. https://www.freedesktop.org/software/systemd/man/systemd-nspawn.html#--property= ↩

  66. "2.3. Modifying control groups Red Hat Enterprise Linux 7". Red Hat Customer portal. https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/7/html/resource_management_guide/sec-modifying_control_groups ↩

  67. Polvi, Alex. "CoreOS is building a container runtime, rkt". CoreOS Blog. Archived from the original on 2019-04-01. Retrieved 12 March 2019. https://web.archive.org/web/20190401013449/https://coreos.com/blog/rocket.html ↩

  68. "LXD". linuxcontainers.org. Retrieved 2021-02-11. https://linuxcontainers.org/lxd/ ↩

  69. Rootless containers with Podman and fuse-overlayfs, CERN workshop, 2019-06-04 https://indico.cern.ch/event/757415/contributions/3421994/attachments/1855302/3047064/Podman_Rootless_Containers.pdf ↩

  70. "Overview — Charliecloud 0.25 documentation". Retrieved 4 October 2020. https://hpc.github.io/charliecloud/ ↩

  71. "Home". katacontainers.io. https://katacontainers.io/ ↩

  72. "Bottlerocket is a Linux-based operating system purpose-built to run containers". https://aws.amazon.com/bottlerocket/ ↩