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XenDesktop Architecture

XenDesktop Architecture

XenDesktop is a complete, end-to-end solution desktop virtualization system that centralizes and delivers virtual desktops to users, which improves data security by centralizing desktop management within the datacenter. XenDesktop is comprised of several Citrix component technologies that provide the essential functionality required to effectively manage, maintain and optimize all desktop connections. Identifying how XenDesktop works and how it fits into a virtual solution is a requirement for Citrix engineers.





Virtual Desktop Components
Engineers must make considerations about the following desktop components prior to integrating a virtual desktop solution:
  • Streamed operating systems
  • Application delivery methods
  • Required privilege level
  • Workload profiles
  • Profile management
  • User data
  • Web Interface




Streamed Operating Systems
Engineers must look at several different factors when deciding whether or not an operating system can be streamed using Provisioning Services, such as:
Network Speed
Streaming an operating system on a network that is already fully utilized in a lower bandwidth network would negatively affect existing systems and the streamed operating systems.
Network Topology
Site locations and site-to-site WAN links affect whether or not streaming an operating system through Provisioning Services should be considered. If the servers running Provisioning Services cannot be located throughout the environment in close proximity on a LAN to where the target devices will be located, or the WAN links are either too slow or too small, then streaming might not be the best option.
If it is determined that an operating system should not be streamed, then it should be installed as a virtual machine with its own virtual hard disk. Installing the operating system on a virtual machine makes the operating system more static. All of the changes will be saved to it, unlike a standard mode disk that can restart the operating system to a clean reference image upon restart.
vDisk Types
The following vDisk image types can stream an operating system:
  • Standard image
  • Private image
  • Difference disk image
Standard Image
Multiple target devices share the same vDisk image. The vDisk is configured in read-only format and any changes made by the target device are stored in a write cache file for the duration of the desktop session.
Benefits
Considerations
  • Many virtual desktops can be deployed from a single vDisk creating a central point of management of the virtual desktop.
  • Desktops are reverted to a consistent state after each restart.
  • Storage requirements are reduced as the write cache is reset after each restart.
  • Any application or system-level automatic updates will start after every restart.
Important: Automatic updates should be disabled at the operating system and application level.
  • Applications that are installed and data that is created by the user during the session is discarded after each restart, which may be unexpected by users.
Private Image
Each target device is provided with an assigned vDisk. The vDisk is configured in read/write format allowing any changes to be saved for further use.
Benefits
Considerations
  • Users are able to completely personalize their desktops and all applications.
  • User data and configurations are saved on the vDisk.
  • vDisk support will become increasingly difficult as every image becomes customized for every user.
  • Private images can require extensive storage


Difference Disk Image
Multiple target devices share the same vDisk image. The vDisk is configured in read-only format and any changes made to the target device are stored in a differential cache file. Upon restart, the differential cache file is saved and reused for future sessions.
Benefits
Considerations
  • Allows for greater level of customization by retaining changes made by each user.
  • The storage savings over the use of private images is significant.
If the base vDisk is ever modified, the user changes are deleted and the user must recreate their customizations.

Application Delivery Methods in a Virtual Desktop Environment
The following application delivery methods are available in a virtual desktop environment:
  • Locally installed application
  • Online application
  • Offline application

Locally Installed Application
Applications can be installed locally on the virtual desktop image.
Benefits
Considerations
  • The applications use the resources of the virtual desktop for processing.
  • Application compatibility with the desktop operating system can be greater than with a server-based model.
  • Applications that are not suitable to run on XenApp should be installed locally on the virtual desktop image. For example, an application that writes registry entries to the local machine hives or one that is not programmed to the Win32 standard
  • The resources configured for the virtual desktop must be sized appropriately to support the operating system and the applications running in it.
Online Application
Applications can be accessed by users as an online application from the XenApp server through use of the Citrix Online plug-in.
Benefits
Considerations
  • Online applications are installed centrally, or streamed to XenApp servers, which can ease application maintenance.
  • Online applications are processed on the XenApp servers, taking advantage of the resources available to the server operating system. This allows for scaling of application performance regardless of the endpoint device in use.
  • Only virtual channel information is sent to the endpoint devices accessing the online applications, which minimizes bandwidth and increases security.
  • Applications must be compatible to run on XenApp servers.
  • Network connectivity must be maintained during the use of online applications.
Offline Application
Applications can be streamed from a file or web server to the virtual desktop. This application delivery method requires users to have the Offline plug-in installed on their desktops.
Benefits
Considerations
  • Offline applications do not have to be installed into the virtual desktop image.
  • Offline applications use the resources of the virtual desktop for processing.
  • Several versions of the same application can be streamed and run at the same time on a virtual desktop.
  • An example of an application that cannot run on XenApp is an application that writes registry entries to the local machine hives or one that is not programmed to the Win32 standard.
  • The resources configured for the virtual desktop must be sized appropriately to support the operating system and the offline applications running in it.
  • Offline applications with dependencies on other offline applications must be planned out as linked profiles and configured as part of the application profiling process.
  • For more information on application streaming, see Citrix Knowledge Base article CTX116414.

Workload Profiles
An engineer can use workload profiles to identify the peak and average resource needs of the applications and the desktop operating systems. This information can then be used to determine the amount of resources that should be assigned to each virtual desktop within the environment to meet the needs of the virtual desktops and the environment during normal and peak usage.
Resource needs can be obtained by gathering performance metrics from the desktop virtualization assessment or by using the performance counters included in the Windows operating system, as well as third-party tools. The data collected should be organized to highlight the peak and average resource needs of all of the applications and the desktop operating system. This information can then be used to determine the amount of resources that should be assigned to each virtual desktop within the environment to meet the overall average needs, while accounting for peak usage periods.
Workload Profile Resource Requirements
The workload profiles for a virtual desktop environment can be determined from the resource requirements of the following items:
  • The operating system that will be delivered within the virtual desktop, including:
    • CPU
    • Memory
    • Disk
  • The enterprise applications that will be running within the virtual desktop operating system, including:
    • The peak and average CPU
    • The peak and average memory
    • The peak and average network
    • The peak and average disk
  • The supporting applications, such as antivirus software, instant messaging, system management applications and monitoring applications, that will be running within the virtual desktop operating system, including:
    • The peak and average CPU
    • The peak and average memory
    • The peak and average network
    • The peak and average disk 

Sizing Virtual Desktops on a Single Host Server


Profile Management
Similar to a XenApp environment, a virtual desktop environment should have a profile management solution in place.
In a virtual desktop environment in which the virtual desktop operating systems are refreshed after each session, user profiles and folders must be saved in a separate, safe location to prevent them from being deleted with the rest of the changes to the virtual desktop.
When delivering a full desktop, certain profile settings need to be saved, including aspects of the desktop interface in addition to any applications that run within the virtual desktop. This includes background colors, font selections and icon sizes.
To manage user profiles within a virtual desktop environment, Citrix Profile management is available to customers who are licensed to use the Advanced, Enterprise or Platinum Edition of XenDesktop. 
Profile management can be used to select specific parts of a profile to be saved in a separate location from the desktop at logon and logoff. Along with mandatory user profiles, this provides a method of saving the personalized user profile settings while decreasing the size of the user profiles.
Environments that cannot utilize a profile management tool, such as Citrix Profile management, must still monitor and manage user profiles.
User Data
Any data that is generated from a user should be redirected and saved outside of the XenApp server, including, but not limited to documents, pictures and PowerPoint files. Engineers must determine how to redirect and store user data in a XenDesktop environment.
In a XenDesktop environment, users are always presented with a complete desktop interface. Therefore, special care must be taken to store settings related to the entire desktop interface in addition to the applications that run within it.
Engineers can use Active Directory Group Policies to redirect user data to a network location where it is stored centrally, backed up and can be accessed from any virtual desktop. Citrix policies can also be used to redirect user data by hiding users' local drives and presenting them with network drives only. 


Web Interface
To access XenDesktop virtual desktop resources, a Web Interface server must be installed to interact with the Desktop Delivery Controllers.
By default, the Web Interface is installed and configured on a Desktop Delivery Controller. If a separate Web Interface server is in place, then the XenDesktop farm must be configured in the XenApp Web site settings through the Access Management Console.
Several Web Interface servers should be deployed to provide redundancy and performance in XenDesktop environments. These Web Interface servers can be load balanced with a device such as NetScaler VPX









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