The architecture of OpenVZ is different from the traditional virtual machines architecture because it always runs the same OS kernel as the host system (while still allowing multiple Linux distributions in individual containers). This single-kernel implementation technology enables running containers with a near-zero overhead. Thus, OpenVZ offer an order of magnitude higher efficiency and manageability than traditional virtualization technologies.
From the point of view of applications and container users, each container is an independent system. This independence is provided by a virtualization layer in the kernel of the host OS. Note that only a negligible part of the CPU resources is spent on virtualization (around 1-2%). The main features of the virtualization layer implemented in OpenVZ are the following:
- A container looks and behaves like a regular Linux system. It has standard startup scripts; software from vendors can run inside a container without OpenVZ-specific modifications or adjustment;
- A user can change any configuration file and install additional software;
- Containers are completely isolated from each other (file system, processes, Inter Process Communication (IPC), sysctl variables);
- Processes belonging to a container are scheduled for execution on all available CPUs. Consequently, CTs are not bound to only one CPU and can use all available CPU power.
A live migration and checkpointing feature was released for OpenVZ in the middle of April 2006. It allows to migrate a container from one physical server to another without a need to shutdown/restart a container. The process is known as checkpointing: a CT is frozen and its whole state is saved to the file on disk. This file can then be transferred to another machine and a CT can be unfrozen (restored) there. The delay is about a few seconds, and it is not a downtime, just a delay.
OpenVZ is free open source software, available under GNU GPL. OpenVZ is the basis of Parallels Virtuozzo Containers, a commercial virtualization solution offered by Parallels.
