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== OS Virtualization ==
From the point of view of applications and [[container]] users, each VE 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]] (CT) 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;
* [[ContainersContainer]]s 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, [[CT]]s are not bound to only one CPU and can use all available CPU power.
{{Main|Checkpointing and live migration}}
A live migration and checkpointing feature was released for OpenVZ in the middle of April 2006. It allows to migrate a VE container from one physical server to another without a need to shutdown/restart a container. The process is known as checkpointing: a CT is freezed 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 unfreezed unfrozen (restored) there. The delay is about a few seconds, and it is not a downtime, just a delay.
Since every piece of the container state, including opened network connections, is saved, from the user's perspective it looks like a delay in response: say, one database transaction takes a longer time than usual, when it continues as normal and user doesn't notice that his database is already running on the another machine.
That feature makes possible scenarios such as upgrading your server without any need to reboot it: if your database needs more memory or CPU resources, you just buy a newer better server and live migrate your container to it, then increase its limits. If you want to add more RAM to your server, you migrate all VEs containers to another one, shut it down, add memory, start it again and migrate all containers back.
[[Category: Concepts]]
[[Category: Technology]]