OpenVZ Philosophy
13
OS Virtualization
From the point of view of applications and Virtual Private Server users, each VPS is an
independent system. This independency is provided by a virtualization layer in the kernel of the
host OS. Note that only an infinitesimal 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:
VPS looks like a normal Linux system. It has standard startup scripts, software from
vendors can run inside VPS without OpenVZ-specific modifications or adjustment;
A user can change any configuration file and install additional software;
Virtual Private Servers are fully isolated from each other (file system, processes, Inter
Process Communication (IPC),
sysctl
variables);
Processes belonging to a VPS are scheduled for execution on all available CPUs.
Consequently, VPSs are not bound to only one CPU and can use all available CPU power.
Network Virtualization
The OpenVZ network virtualization layer is designed to isolate VPSs from each other and from
the physical network:
Each VPS has its own IP address; multiple IP addresses per VPS are allowed;
Network traffic of a VPS is isolated from the other VPSs. In other words, Virtual Private
Servers are protected from each other in the way that makes traffic snooping impossible;
Firewalling may be used inside a VPS (the user can create rules limiting access to some
services using the canonical
iptables
tool inside the VPS). In other words, it is possible
to set up firewall rules from inside a VPS;
Routing table manipulations are allowed to benefit from advanced routing features. For
example, setting different maximum transmission units (MTUs) for different destinations,
specifying different source addresses for different destinations, and so on.
Templates
An OS template in OpenVZ is basically a set of packages from some Linux distribution used to
populate one or more VPSs. With OpenVZ, different distributions can co-exist on the same
hardware box, so multiple OS templates are available. An OS template consists of system
programs, libraries, and scripts needed to boot up and run the system (VPS), as well as some
very basic applications and utilities. Applications like a compiler and an SQL server are usually
not included into an OS template.
For detailed information on OpenVZ templates, see the
Understanding Templates
section.