Decisions, decisions: VDI storage considerations and choices
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The past 18 months have seen huge investment by VMware within the EUC space with the arrival of Sanjay Poonen and...
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Horizon 6 (with View), which introduced application publishing in the Advanced edition. Finally, there was an emerging contender to the heavyweight Citrix XenApp.
Following this investment from VMware, I have seen more potential customers who are looking at virtualizing desktops and applications. But not every physical computer is a virtualization candidate. To determine this, you have to do a desktop assessment.
What is a desktop assessment?
First of all, I want to define what is meant by a desktop assessment. I use a piece of centralized software that collates information from remote agents installed on the various end-user devices that are candidates for VDI.
Switching the operating system?
After the desktop assessment, you need to decide if you are staying with the same operating system or moving to a new one.
If you perform a VDI assessment on a desktop operating system that is going to be replaced with a newer version, what value are you really obtaining? The answer is not a lot. The applications will most likely require updating to support the new OS and this in turn leads to different requirements for compute and storage requirements.
Savings from the same operating system
If you are going to keep the same operating system, you will get more value from the desktop assessment. However, it's worth keeping in mind that the results from the desktop assessment often overinflate your compute metrics.
- Compute resources used by an in-guest anti-virus are likely to be offloaded to a host-based alternative.
- Compute and storage resources for Windows updates will often be negated by VDI tools such as PVS, MCS and linked clones.
- Applications installed by the end user will most likely be removed from the master image.
- The VDI master image will be optimized with services, widgets and applications being disabled or uninstalled.
This can be viewed as a positive thing as you can often show a slightly higher consolidation ratio per physical host.
What peripherals do users have?
This is where desktop assessments come into their own. Most IT departments I have spoken to say they are aware of what applications and devices their employees are using, but I find that hard to believe.
Desktop assessments will inform you what parallel, serial and USB devices are connected to the user's computer. This gives you the visibility to determine whether a particular user's device is appropriate for VDI.
A desktop assessment helps clarify the types of applications, software and devices being used. However, they often fall short in a number of areas:
- Application dependencies: Determine why you have five different versions of Java installed.
- They often look to see if an executable is launched, not whether an application is used to read or edit a document which can have a huge effect on license cost.
- Application readiness and/or virtualization assessment: Will the application work on a certain operating system and is it capable of being virtualized?
Often this area is overlooked and requires a large effort from a separate work stream outside of the desktop assessment. This information from any desktop assessment can be used as a starting point.
Most desktop assessments rely on an in-guest agent on the end device to capture metrics and pass them back to a central collection repository. What happens when you are waiting for that agent to start? The answer is nothing; you miss collecting data on anything that happens prior to the agent starting.
When the agent does start, the metrics collected for login or logoff times can be skewed by group policy applied to the computer object.
Ask yourself how often a new organizational unit is created for VDI deployments.
How to get accurate storage numbers
We have already established that the in-guest agent doesn't start until the OS is ready, so we have missed boot IOPS metrics.
Desktop assessments have the ability to capture steady state information, which is OK as long as there are no other bottlenecks skewing the provided information. Examples include:
- Is paging occurring, causing disk I/O to increase?
- Is the limiting factor the hard drive itself, and if unleashed from a 7200 rpm SATA hard drive, what IOPS would be consumed?
- Are anti-virus scans causing peaks in provided disk I/O information?
What is the value?
For me, there are four things that hold high value in a desktop assessment for VDI.
First, it enables you to take a bird's eye view of which users are virtualization candidates and when items, such as peripherals, are taken into consideration.
It also provides a classification for users -- low, medium or high -- for resource consumption.
Next, desktop assessments also allow you to determine concurrent logins and logoffs, which helps to calculate storage sizing requirements.
Lastly, desktop assessments uncover what applications are being used.
The desktop assessment does have some value in the VDI world, but it isn't a panacea to provide you with everything you need to know on your journey to VDI. Most of the value comes from a pilot and load testing with products such as LoginVSI to determine the density of users per host.
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