Virtuozzo is a proprietary operating system virtualization product produced by SWsoft, Inc. A version that supports Linux has been available since 2001. Virtuozzo creates multiple, isolated virtual environments (VEs, otherwise known as Virtual Private Servers , or VPS) on a single physical server . This allows the sharing of hardware, systems management effort, and sometimes software licenses among different workloads.
The VPS behaves in most respects as if it were a stand-alone server. Each VPS has its own superuser (Root or Administrator), set of users/groups, IP address(es), processes, files, applications, system libraries and configuration files. The VPS is completely accessible via the network. Due to virtualized network stack, a VPS owner can have his own routing tables and firewall (iptables) rules.
Virtuozzo can support tens to hundreds of Virtual Private Servers (VPS) on a single server due to its use of operating system-level virtualization.
Virtuozzo virtualizes at the operating system level, rather than at the hardware level as other Virtual Machine (VM) products frequently do. Whereas VMs attempt to virtualize "a complete set of hardware," VPSs represent a "lighter" abstraction, virtualizing instead "an operating system instance". All VPSs run atop a single operating system kernel. The VPS mechanism multiplexes this one OS kernel to look like multiple OS (and server) instances, especially from the perspective of running applications, users, and network services. Virtuozzo is based on OpenVZ , and its concepts are similar to other operating system-level virtualization implementations, including Solaris Containers and FreeBSD Jail.
Because they virtualize less, VPSs impose lower overhead than VMs. As a result, more VPSs can be supported on a given server. Proponents occasionally claim "thousands of VPSs per server" in test situations to determine the upper limits of the technology. Virtuozzo supports servers with up to 64 x86 CPUs and 64 GB of RAM, but 1-4 CPU systems are far more common in practice.
A live migration and checkpointing feature was released for OpenVZ in the middle of April 2006. It allows to migrate a VPS from one physical server to another without a need to shutdown/restart a VPS. The process is known as checkpointing: a VPS is frozen and its whole state is saved to the file on disk. This file can then be transferred to another machine and a VPS can be unfrozen (restored) there. The delay is about a few seconds, and it is not a downtime, just a delay. Since every piece of VPS state, including opened network connections, is saved, from the user's perspective it looks like a delay in response: one database transaction takes a longer time than usual, when it continues as normal the user doesn't notice that his database is already running on 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 VPS to it, then increase its limits. If you want to add more RAM to your server, you migrate all VPSs to another one, shut it down, add memory, start it again and migrate all VPSs back.
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