In accordance with Moore's Law, today's accelerated development of information technology and the penetration of virtualization technology makes it possible to break the traditional and typically inefficient "one-application-server-one" approach. Different applications have different kind of workloads imposed on the servers. While a typical file server has only a few percent of CPU load, in case of a hard-loaded database server up to 80% or more CPU load may occur. These workloads can be easily distributed and balanced in virtualized environment.

Using virtualization technologies make available the following advantages compared to the traditional approach:
Virtualizációs technológiák alkalmazásával az alábbi előnyök érhetők el szemben a tradícionális megközelítéssel:

  • Optimizing the utilization of hardware resources
  • Less hardware is required for these systems to serve
  • Lower data center costs (electricity, air conditioning, etc.).
  • Reduced space requirements in the data center
  • Decreased the cost and time-frame of planned / unplanned downtime
  • Increase availability
  • Hardware independent
  • Easier resource-management

Brief overview of the different aspects of virtualization:

Application-level virtualization

  • The virtualiazation is realized not in OS-level
  • Cygwin
    • Realizing Posix/GNU programming environment in Windows OS
    • Porting softwares developed in UNIX/Linux environment to Windows OS
  • Wine
    • Realizing Windows APIs and some kernel functions in Linux and X11 environment

Operating system-level virtualization

  • Realizing several program executing environment separated from each other, within one OS
  • It complements the base OS to be able to create multiple VE* within itself
  • Actually these VEs are the virtual copies of the same OS
  • Unable to run a different OS
  • Only 1-3% performance overhead
  • Virtuozzo (Virtual Private Server)
  • Jails (limited, secure access to the resources of the base OS)

* VE: Virtual Environment


  • Simulating a physical machine’s structure partly or wholly on another physical machine
  • C64, Amiga, Super Nintendo, Playstation etc. emulators
  • Real time transformation of the virtual environment instructions into the physical environment instructions by software
  • No need for any modification on any OS
  • Any hardware component can simulated by its use (even if it does not exist)
  • Performance problems may occur

API Emulation

  • It allows the host OS to run different OSs in the virtual system
  • Host OS APIs (almost) one-to-one mapping of the guest OS with the appropriate library
  • VMware
  • MS VirtualPC and MS Virtual Server 2005


  • Virtual machines can directly communicate with the hardware
  • The host OS provides direct access to the hardware via API
  • The guest OS must be prepared for this (porting)
  • Low ~5% performance overhead
  • User Mode Linux
    • Virtualizing Linux systems in Linux environment
    • Part of the Linux-kernel for today
  • XEN
    • Virtualizing Linux, FreeBSD, Windows XP, Windows 2003 Server etc.
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