Operating system-level virtualization

Operating system-level virtualization is a server virtualization method where the kernel of an operating system allows for multiple isolated user-space instances, instead of just one. Such instances (often called containers, VEs, VPSs or jails) may look and feel like a real server, from the point of view of its owner. On Unix systems, this technology can be thought of as an advanced implementation of the standard chroot mechanism. In addition to isolation mechanisms, the kernel often provides resource management features to limit the impact of one container's activities on the other containers.

Uses

Operating system-level virtualization is commonly used in virtual hosting environments, where it is useful for securely allocating finite hardware resources amongst a large number of mutually-distrusting users. System administrators may also use it, to a lesser extent, for consolidating server hardware by moving services on separate hosts into containers on the one server.

Other typical scenarios include separating several applications to separate containers for improved security, hardware independence, and added resource management features.

OS-level virtualization implementations that are capable of live migration can be used for dynamic load balancing of containers between nodes in a cluster.

Advantages and disadvantages

Overhead

This form of virtualization usually imposes little or no overhead, because programs in virtual partition use the operating system's normal system call interface and do not need to be subject to emulation or run in an intermediate virtual machine, as is the case with whole-system virtualizers (such as VMware and QEMU) or paravirtualizers (such as Xen and UML). It also does not require hardware assistance to perform efficiently.

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 OS such as Windows cannot be hosted. This limitation is partially overcome in Solaris by 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. (a Linux branded zone was also announced but later abandoned).

Storage

Some operating-system virtualizers provide file-level copy-on-write mechanisms. (Most commonly, a standard file system is shared between partitions, and partitions which 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

Mechanism	Operating system	License	Available since/between	Features
				File system isolation	Copy on Write	Disk quotas	I/O rate limiting	Memory limits	CPU quotas	Network isolation	Partition checkpointing and live migration
chroot	most UNIX-like operating systems	Proprietary BSD GNU GPL CDDL	1982	Partial[1]	No	No	No	No	No	No	No
iCore Virtual Accounts	Windows XP	Proprietary/Freeware	2008	Yes	No	Yes	No	No	No	No	No
Linux-VServer (security context)	Linux	GNU GPL v.2	2001	Yes	Yes	Yes	Yes [2]	Yes	Yes	Partial[3]	No
LXC	Linux	GNU GPL v.2	2008	Partial[4]	Partial. Yes with Btrfs.	Partial. Yes with LVM or Disk quota.	Yes	Yes	Yes	Yes	No
OpenVZ	Linux	GNU GPL v.2	2005	Yes	No	Yes	Yes [5]	Yes	Yes	Yes[6]	Yes
Parallels Virtuozzo Containers	Linux, Windows, Mac OS X	Proprietary	2001	Yes	Yes	Yes	Yes [7]	Yes	Yes	Yes[6]	Yes
Container/Zone	Solaris and OpenSolaris	CDDL	2005	Yes	Partial. Yes with ZFS	Yes	No	Yes	Yes	Yes[8]	No[9]
FreeBSD Jail	FreeBSD	BSD	1998	Yes	Yes (ZFS)	Yes [10]	No	Yes [11]	Partial [12]	Yes	No
sysjail	OpenBSD, NetBSD	BSD	- no longer supported as of 03-03-2009	Yes	No	No	No	No	No	Yes	No
WPARs	AIX	Proprietary	2007	Yes	No	Yes	Yes	Yes	Yes	Yes[13]	Yes[14]
HP SRP	HPUX	Proprietary	2007	Yes	No	—	Yes	Yes	Yes	Yes	Yes
Sandboxie	Windows	Shareware	?	?	?	?	?	?	?	?	?

Notes

1. ^ Root user can easily escape from chroot. Chroot was never supposed to be used as a security mechanism.^{[according to whom?]} [1]
2. ^ Utilizing the CFQ scheduler, you get a separate queue per guest.
3. ^ Networking is based on isolation, not virtualization.
4. ^ Due to lack of user name space separation in the Linux kernel it is currently possible to evade from linux containers
5. ^ Available since 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. See OpenVZ wiki: I/O priorities for VE for details.
6. ^ ^{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.
7. ^ Available since version 4.0, January 2008.
8. ^ See OpenSolaris Network Virtualization and Resource Control and Network Virtualization and Resource Control (Crossbow) FAQ for details.
9. ^ Cold migration (shutdown-move-restart) is implemented.
10. ^ Check the "allow.quotas" option and the "Jails and File Systems" section on the FreeBSD jail man page for details.
11. ^ http://wiki.freebsd.org/Hierarchical_Resource_Limits
12. ^ Check the "cpuset.id" option on the FreeBSD jail man page and the cpuset command. It's not a full cpu quota support because it constraints one or more processes to a given set/list of processors (cores) but it doesn't control the absolute processor/core related utilization.
13. ^ Available since TL 02. See [2] for details.
14. ^ See [3]

See also

• Platform virtualization
• Application virtualization
• Storage Hypervisor
• Hypervisor
• Portable application creators

External links

• An introduction to Virtualization
• A short intro to three different virtualization techniques