Do you Really Need That Service on Your VDI Master Image?

By John Kelly, Yinglong Jiang

As the rate of adoption of desktop virtualization continues to increase, the advantages of the desktop virtualization paradigm are becoming increasingly attractive to organizations who are trying to solve multiple problems, including increased consumerization / BYOD, static (or reduced) budgets and increasing organizational and regulatory complexities. One of the primary questions asked in this context is around the total-cost-of ownership (TCO) of a virtual desktop infrastructure (VDI) environment.

TCO is often analyzed from 2 perspectives: operational expenditure (OpEx) and capital expenditure (CapEx).

• OpEx reductions are achieved through many different mechanisms but one of the primary mechanisms is through the reduced personnel resource requirements for activities such as image management: the centralized VDI model allows IT administrators to modify (e.g. patch) a base Windows OS image once, with this modification propagating out to all users of that shared image. This compares very favorably with the requirement to modify multiple physical desktops.
• CapEx is usually primarily influenced by the “up-front” investment required in hardware and software. A big influencing factor in relation to this investment is “user density” – how many virtual desktops can be hosted by a particular piece of server, storage or network hardware while providing users of those desktops with a good end-user experience?

However, it is important to be conscious of the interdependence between centralized image management and user density: because of the shared nature of a VDI environment (with shared compute, storage and network resources), incremental activities (e.g. Windows services) that are “baked into” the base image will cause hardware resource utilization by all virtual desktops. This incremental resource utilization, when aggregated up to the level of the shared resource (i.e. server, storage or network) will cause that resource to reach maximal capacity at a user density that is less that the density that would be achieved without the incremental activity, resulting in larger CapEx requirements. So from a TCO perspective, centralized image management is one of the biggest advantages in terms of reduced OpEx in a VDI environment but it also presents one of the biggest risks in terms of potential for increased CapEx. Read on for a description of some performance analysis and characterization (PAAC) work carried out in the Dell Desktop Virtualization Solutions (DVS) engineering organization as an example of how differing master image configurations can result in significant differences in the hardware resource utilization of a VDI environment….

Before we introduce the results that we saw, it is first necessary to take a technical detour into 3 tools and technologies that are commonly used in VDI and enterprise IT environments.

1. Login VSI is the de-facto industry standard for load-testing VDI environments. This load-testing activity is implemented by running a representative set of applications that use a representative set of data files for these applications. Login VSI 4 was released during 2013; this tool has significant additional functionality in comparison with version 3; in addition, some of the “plumbing” of the test tool has changed, with many data files now being copied to the local virtual desktop when required, as opposed to the version 3 approach of staging all files on the virtual desktop throughout the test.
2. Microsoft Volume Shadow Copy Service (VSS) facilitates backups and point-in-time copies in recent versions of Microsoft operating systems by providing applications with advanced capabilities such as the capability to flush partially committed data from memory. VSS functionality is provided using a Windows service.
3. VMware View Persona Management is an integrated profile management tool for VMware View environments that optimizes the handling of user profiles at login and logout for View virtual desktops and dynamically synchronizes the profiles with a remote profile repository using VSS. View Persona Management functionality is primarily provided through an Active Directory Group Policy Object (GPO) and VSS.

In order to assess the hardware resource utilization impact of the interaction of the above tools and technologies, PAAC work was carried out using a VMware View environment with a constant number of virtual desktops running a Login VSI 4 medium workload; resource utilization was monitored in four scenarios:

1. View Personas (and VSS) disabled
2. View Personas (and VSS) enabled.
3. View Personas (and VSS) disabled and reverting to Login VSI 3 approach of staging data files on virtual desktop rather than on network share.
4. View Personas (and VSS) enabled and reverting to Login VSI 3 approach of staging data files on virtual desktop rather than on network share.

The purpose of the performance analysis work described in scenarios 3. and 4. above is to analyze the impact of incremental activities that result from the changed file copy activity; an example of these incremental activities is additional VSS related activities resulting from the VSS service having to perform copies more frequently due to the data on the virtual desktop changing more frequently. Although the Login VSI and file copy activities described in the above scenarios relate to a very specific environment, the principles in relation to image optimization have much broader relevance.

Table 1 below summarizes the results obtained from the analysis described above in terms of both host CPU utilization and disk IOPS per user. For both of these resource utilization parameters, the results shown are normalized with respect to resource utilization with personas / VSS disabled and locally staged data files i.e. that scenario is regarded as having a resource utilization of 100 for both CPU utilization and disk IOPS and all other results are relative to that 100 baseline level.

Table 1: CPU and Disk IOPS Comparison

The following significant points of interest can be derived from the above table:

1. The introduction of VMware View Personas (incorporating VSS usage) results in a significant increase in both CPU utilization and disk IOPS when compared to the baseline.
2. The different level of file copy activity that results from continuous file copies onto the master image in Login VSI 4 results in increased disk IO activity, particularly when combined with Personas / VSS (resulting in a 38% increase from the baseline as opposed to the 27% increase without Personas / VSS).

The increased IOPS generated by the Personas / VSS scenario is confirmed by per-desktop analysis of virtual desktop resource utilization using the Liquidware Labs Stratusphere UX tool; this analysis consistently shows VSS as one of the largest consumers of CPU and disk IO hardware resources on each virtual desktop for desktops with VSS enabled.

So what does the above information mean? It certainly doesn’t mean that Login VSI or VMware View Personas are poor technologies! The faster login times provided by the Personas approach of only downloading files and registry keys that are required at login will be very attractive to many organizations, for example. In addition, the scenario described above is very particular to an internal load-testing environment (although scenarios where scheduled jobs frequently copy files to the user desktop could also occur in a real production environment). The significance of the above information is to emphasise the importance of the following best practices:

1. Optimize your base image to ensure it doesn’t include any unneeded services or scheduled tasks.
2. Carry out a proof-of-concept or pilot VDI implementation to ensure that you are aware of the implications of your base-image related configuration decisions and to allow the implications of “what-if” scenarios to be considered.

The best way to implement the above best practices? Engage with Dell for a comprehensive analysis of your environment,including activities such as a Blueprint Assessment and Proof-Of-Conceptthereby ensuring an optimized deployment of a Dell desktop virtualization solution in your organization and accelerating your journey to a successful VDI implementation. Also, watch this space for some further fascinating data points around image optimization and its impact…