7 min read

(For more resources related to this topic, see here.)

Sizing the servers

There are a number of tools and guidelines to help you to size Citrix VIAB appliances. Essentially, the guides cover the following topics:

  • CPU
  • Memory
  • Disk IO
  • Storage

In their sizing guides, Citrix classifies users into the following two groups:

  • 4kers
  • Knowledge workers

Therefore, the first thing to determine is how many of your proposed VIAB users are task workers, and how many are knowledge workers?

Task workers

Citrix would define task workers as users who run a small set of simple applications, not very graphical in nature or CPU- or memory-intensive, for example, Microsoft Office and a simple line of business applications.

Knowledge workers

Citrix would define knowledge workers as users who run multimedia and CPU- and memory-intensive applications. They may include large spreadsheet files, graphics packages, video playback, and so on.


Citrix offers recommendations based on CPU cores, such as the following:

  • 3 x desktops per core per knowledge worker
  • 6 x desktops per core per task user
  • 1 x core for the hypervisor

These figures can be increased slightly if the CPUs have hyper-threading.

You should also add another 15 percent if delivering personal desktops.

The sizing information has been gathered from the Citrix VIAB sizing guide PDF.

Example 1

If you wanted to size a server appliance to support 50 x task-based users running pooled desktops, you would require 50 / 6 = 8.3 + 1 (for the hypervisor) = 9.3 cores, rounded up to 10 cores.

Therefore, a dual CPU with six cores would provide 12 x CPU cores for this requirement.

Example 2

If you wanted to size a server appliance to support 15 x task and 10 x knowledge workers you would require (15 / 6 = 2.5) + (10 / 3 = 3.3) + 1 (for the hypervisor) = 7 cores.

Therefore, a dual CPU with 4 cores would provide 8 x CPU cores for this requirement.


The memory required depends on the desktop OS that you are running and also on the amount of optimization that you have done to the image.

Citrix recommends the following guidelines:

  • Task worker for Windows 7 should be 1.5 GB
  • Task worker for Windows XP should be 0.5 GB
  • Knowledge worker Windows 7 should be 2 GB
  • Knowledge worker Windows XP should be 1 GB

It is also important to allocate memory for the hypervisor and the VIAB virtual appliance. This can vary depending on the number of users, so we would recommend using the sizing spreadsheet calculator available in the Resources section of the VIAB website. However, as a guide, we would allocate 3 GB memory (based on 50 users) for the hypervisor and 1 GB for VIAB. The amount of memory required by the hypervisor will grow as the number of users on the server grows.

Citrix also recommends adding 10 percent more memory for server operations.

Example 1

If you wanted to size a server appliance to support 50 x task-based users, with Windows 7, you would require 50 x 1.5 + 4 GB (for VIAB and the hypervisor) = 75 GB + 10% = 87 GB.

Therefore, you would typically round this up to a 96 GB memory, providing an ideal configuration for this requirement.

Example 2

Therefore, if you wanted to size a server appliance to support 15 x task and 10 x knowledge workers, with Windows 7, you would require (15 x 1.5) + (10 x 2) + 4 GB (for VIAB and the hypervisor) = 75 GB + 10% = 51.5 GB.

Therefore, a 64 GB memory would be an ideal configuration for this requirement.

Disk IO

As multiple Windows images run on the appliances, disk IO becomes very important and can often become the first bottleneck for VIAB.Citrix calculates IOPS with a 40-60 split between read and write OPS, during end user desktop access.Citrix doesn’t recommend using slow disks for VIAB and has statistic information for SAS 10 and 15K and SSD disks.The following table shows the IOPS delivered from the following disks:

Hard drive RPM












The following table shows the IOPS required for task and knowledge workers for Windows XP and Windows 7:

Desktop IOPS

Windows XP

Windows 7

Task user



Knowledge user



Some organizations decide to implement RAID 1 or 10 on the appliances to reduce the chance of an appliance failure. This does require many more disks however, and significantly increases the cost of the solution.


SSD is becoming an attractive proposition for organizations that want to run a larger number of users on each appliance. SSD is roughly 30 times faster than 15K SAS drives, so it will eliminate desktop IO bottlenecks completely. SSD continues to come down in price, so can be well worth considering at the start of a VIAB project.

SSDs have no moving mechanical components. Compared with electromechanical disks, SSDs are typically less susceptible to physical shock, run more quietly, have lower access time, and less latency. However, while the price of SSDs has continued to decline, SSDs are still about 7 to 8 times more expensive per unit of storage than HDDs.

A further option to consider would be Fusion-IO, which is based on NAND flash memory technology and can deliver an exceptional number of IOPS.

Example 1

If you wanted to size a server appliance to support 50 x task workers, with Windows 7, using 15K SAS drives, you would require 175 / 10 = 17.5 users on each disk, therefore, 50 / 17. 5 = 3 x 15K SAS disks.

Example 2

If you wanted to size a server appliance to support 15 x task workers and 10 knowledge workers, with Windows 7, you would require the following:

  • 175 / 10 = 17.5 task users on each disk, therefore 15 / 17.5 = 0.8 x 15K SAS disks
  • 175 / 20 = 8.75 knowledge users on each disk, therefore 10 / 8.75 = 1.1 x 15K SAS disks

Therefore, 2 x 15K SAS drives would be required.


Storage capacity is determined by the number of images, number of desktops, and types of desktop. It is best practice to store user profile information and data elsewhere.

Citrix uses the following formula to determine the storage capacity requirement:

2 x golden image x number of images (assume 20 GB for an image)

  • 70 GB for VDI-in-a-Box
  • 15 percent of the size of the image / desktop (achieved with linked clone technology)

Example 1

Therefore, if you wanted to size a server appliance to support 50 x task-based users, with two golden Windows 7 images, you would require the following:

  • Space for the golden image: 2 x 20 GB x 2 = 80 GB
  • VIAB appliance space: 70 GB
  • Image space/desktop: 15% x 20 GB x 50 = 150 GB
  • Extra room for swap and transient activity: 100 GB
  • Total: 400 GB
  • Recommended: 500 GB to 1 TB per server

We have already specified 3 x 15K SAS drives for our IO requirements. If those were 300-GB disks, they should provide enough storage.

This section of the article provides you with a step-by-step guide to help you to build and configure a VIAB solution; starting with the hypervisor install.

It then goes onto to cover adding an SSL certificate, the benefits of using the GRID IP Address feature, and how you can use the Kiosk mode to deliver a standard desktop to public access areas.

It then covers adding a license file and provides details on the useful features contained within Citrix profile management. It then highlights how VIAB can integrate with other Citrix products such as NetScaler VPX, to enable secure connections across the Internet and GoToAssist, a support and monitoring package which is very useful if you are supporting a number of VIAB appliances across multiple sites. ShareFile can again be a very useful tool to enable data files to follow the user, whether they are connecting to a local device or a virtual desktop. This can avoid the problems of files being copied across the network, delaying users.

We then move on to a discussion on the options available for connecting to VIAB, including existing PCs, thin clients, and other devices, including mobile devices. The chapter finishes with some useful information on support for VIAB, including the support services included with subscription and the knowledge forums.

Installing the hypervisor

All the hypervisors have two elements; the bare metal hypervisor that installs on the server and its management tools that you would typically install on the IT administrator workstations.

Bare Metal Hypervisor

Management tool

Citrix XenServer


Microsoft Hyper-V

Hyper V Manager

VMware ESXi

vSphere Client

It is relatively straightforward to install the hypervisor. Make sure you enable linked clones in XenServer, because this is required for the linked clone technology. Give the hypervisor a static IP address and make a note of the administrator’s username and password.

You will need to download ISO images for the installation media; if you don’t already have them, they can be found on the Internet.

Subscribe to the weekly Packt Hub newsletter

* indicates required


Please enter your comment!
Please enter your name here