Shared kernel virtualization, also called operating system virtualization or system level virtualization, takes advantage of the unique ability of UNIX and Linux to share their kernels with other processes on the system. This shared kernel virtualization is achieved by using a feature called change root (chroot). The chroot feature changes the root file system of a process to isolate it in such a way as to provide some security. It (chroot) is often called a chroot jail or container-based virtualization. A chrooted program, set of programs, or entire system in the case of shared kernel virtualization is protected by setting up the chrooted system to believe that it is a standalone machine with its own root file system.
The chroot mechanism has been enhanced to mimic an entire file system so that an entire system can be chrooted, hence creating a VM. The technical advantages and disadvantages of shared kernel virtualization are listed next:
Enhanced Security and Isolation
Higher Density of Virtualized Systems
Host Kernel and Guest Compatibility
The chroot system offers much in the way of enhanced security features and isolation; however, the greatest advantages of shared kernel virtualization are not in its security, although that's certainly important to consider, but in its performance. With this kind of virtualization, you'll get native performance for each individual system. Not only does each system perform at native speeds, but you can also have more than the standard number of VMs on a host system. By standard number, we mean the number that you could logically have on a host system if you used memory as the limiting factor—leaving 1GB for the host and taking the rest of the RAM for VMs.
The limit of the number of chrooted systems you can have on a host system more closely resembles a standalone system supporting multiple applications. If you think of each chroot system as an application instead of a VM, you'll more accurately allocate resources and enjoy performance that surpasses many other types of virtualization.
The disadvantage of shared kernel virtualization is a big one: All VMs have to be compatible with your running kernel. In other words, you can't run Windows operating systems, Solaris, Mac OS X, or any other operating system that couldn't run your system's kernel on its own. Major web hosting providers have run this scenario for years so that customers get their own virtual server for their hosting needs. They don't know that the system is virtual, nor can they contact the host system through their VM.
Solaris Containers (Zones)
Solaris 10 comes with built-in virtualization. The Solaris 10 operating system, itself, is known as the Global Zone. Solaris Zones are actually BSD jails, each with its own virtual root that mimics a complete operating system and file system. When you create a new zone, a full file system is copied to the new zone directory. Each zone sees only its own processes and file systems. The zone believes that it is a full, independent operating system; only the Global Zone has any knowledge of virtualization.
Each zone essentially creates a clean sandbox in which you may install applications, provide services, or test patches. Solaris zones are a scalable, enterprise-level virtualization solution providing ease of use and native performance.
We use the OpenVZ kernel on my personal Linux server system. The OpenVZ kernel is optimized for virtualization and proves to be extremely efficient at handling VM performance for other virtualization products as well.
On my personal Linux server system, we run VMware Server, Sun's xVM, and QEMU. Before we installed the OpenVZ kernel, we had many CPU-related performance problems with some of my VMs. OpenVZ is similar to Solaris Zones except that you can run different Linux distributions under the same kernel. Various distribution templates are available on the OpenVZ website at www.openvz.org.