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Effectively calculating Storage Requirements: HP ProLiant and Citrix VDI-in-a-Box

Author:  Frank Anderson, Citrix Systems


HP ProLiant servers & Citrix VDI-in-a-Box combine to allow low-cost easy to deploy Desktop Virtualization solutions. By providing simple and efficient storage options, combining these product sets create turnkey solutions that makes VDI deployments simple efficient and easy to manage.


HP Client Virtualization SMB Reference Architecture has been calculated for a very solid and easy to deploy experience. Within the document you will find all of the details around the extensive testing, including core data points around CPU, server memory and IOPS.  However, for those of you that want to better understand the different storage characteristics of the solution, we are providing the following information for determining storage requirements and understanding your individual use cases.


Start by profiling your user needs, pooled vs. dedicated desktops


Pooled desktops allow multiple virtual desktops to run from a single, centralized virtual desktop image.  By using a centralized virtual desktop image, administrators have the option to simply install all required business applications directly on that golden image, which is then referenced in Citrix VDI-in-a-Box as Pooled Desktops with Installed Applications. The pooled desktop and installed application model can be implemented quickly and with a minimal amount of infrastructure using differencing disks.


Personalized desktops are created from a published Windows 7 image through the VDI-in-a-Box administrative interface. They leverage the simplicity of pooled desktop management while enabling the customization of a personal vDisk. Administrators can customize and maintain the golden image with applications, just as in pooled desktops. In addition, users can install their own applications in their personal vDisk. When the published golden image is updated, the base image of all personal desktops is automatically updated with the administrator’s changes while the user-installed applications, profiles, and data on the personal vDisk remain intact.


The advantage of pooled desktops      

Pooled desktops use only about 15% of the size of a golden image per desktop. This savings is due to the implementation of pooled desktops through “differencing disks’” in which desktops share a common master image, reducing storage capacity requirements. With differencing disks, when desktops are provisioned, a separate, small child disk is created. All changes to the desktop—such as registry changes or installation of new applications—are stored in its difference disk. All reads use the parent disk or the golden image itself. The difference disk grows with use over time.

The impact of dedicated desktops       

For non-persistent desktops that are refreshed periodically, these difference disks are typically very small, usually about 10-15% of the golden image. This reference architecture assumes that pooled desktops are refreshed frequently (at least weekly or bi-weekly), and that user and profile data is stored externally. If that is not the case, or if applications require additional write space, you should allocate more than 15% of the “golden” image size per pooled desktop, or consider the provisioning of personalized desktops.


Doing the math (putting it together)

Note, you can cheat here, as Citrix makes available an on-line VDI-in-a-Box 5.1 Server Sizing Calculator that follows these formulas to predict the amount of storage required in a deployment that will work for 90% of our user base.  So… the details below are for the 10%.


Calculating the required storage capacity follows several basic formulas. To estimate the storage needed, you’ll first need to know the number of golden images, the size of each golden image, the number of pooled desktops, the number of personal desktops, and the size of the personal vDisk to be provisioned for each personal desktop. The paragraphs below describe the formulas used to estimate the amount of storage needed.


The first formula estimates the amount of storage needed for golden images:

Formula 1: 2 X (Size of Golden image) X (Number of Golden Images) =

Disk space (GB) required for Golden Images

For example, if there are two 20GB golden images, the formula is as follows:

Example 1: 2 x 20GB x 2 Golden Images = 80GB


The second formula determines how much space the pooled desktops consume. The rule of thumb is that each linked clone takes up roughly 15% of the golden image size. Assuming a 20GB golden image, the linked clone storage calculation for 105 pooled desktops is as follows:

Formula 2: 15% X (Size of Golden image) X (Number of Pooled Desktops) =

 Disk space (GB) for Pooled Desktops

Example 2: 15% of 20GB x 105 Pooled Desktops = 315GB


The third formula determines how much space is required for personalized desktops. Personal desktops are assigned using a vDisk template that varies in size from 4GB to 60GB (the default is 8GB). Using the 8GB default, the storage calculation for 45 personal desktops would be as follows:

Formula 3: (Size of VHD Template) X (Number of Personal Desktops) =

 Disk space (GB) for Personal Desktops

Example 3:  8GB x 45 Personal Desktops = 360GB


The fourth and final formula determines the amount of swap space to configure. This value represents the memory allocated for each virtual desktop times the total number of virtual desktops configured:

Formula 4: (VM RAM Size) X (Number of VMs) =

Disk space (GB) for swap        

Example 4:  (2GB x 150 Pooled or Personal Desktops) = 300GB



Please refer to the reference architecture for a detailed description of the solution including the testing environment and storage design.



So what’s the bottom line?


By following the recommended guidelines for this pre-validated solution, you can estimate how many servers and what software licenses you need to support various user workloads. And because the solution follows a building-block approach, it scales on demand with no re-architecture or expensive infrastructure upgrades. We recommend that you assess the exact end-user environment to eliminate over-provisioning either manually in small offices or by leveraging services from HP or HP Partners.


To access this entire whitepaper click here; or more information on HP-Citrix client virtualization reference architectures, visit

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