Virtual Observer

Building a VDI Environment on Shared Storage

Storage is at the heart of the problems some companies have in deploying VDI.

Last time, I talked about the challenges that remain around desktop virtualization: too many platform choices, the explosion in the number of end-user devices, the confusion caused by cloud offerings, and the lack of a breakout management solution for desktop virtualization.

Let's dive deeper into the granddaddy solution for desktop virtualization: server-hosted virtual desktops, traditionally called "VDI." VDI has often promised more than it delivered, due to stubborn complexity, performance and cost challenges. Chief among these has been the high up-front capital costs and subsequent inefficiencies of the storage platforms deployed to support it--and storage costs are at the heart of the VDI-ROI equation.

At first glance, it seems obvious that the consolidation, efficiency and mobility benefits of server virtualization should translate easily to VDI. But storage costs complicate that picture. Essentially, it all comes down to density. In order to justify VDI, desktop VMs must be deployed at much higher densities than server VMs--at least five to 10 times higher in many cases. But dense VM deployments tend to expose performance problems and edge conditions throughout the virtual IT stack, from memory and CPU contention to I/O bottlenecks.

Of course, high densities in VDI are only possible because desktop workloads are intermittent and variable. Unfortunately, this also makes them difficult to model and prone to wild load spikes. Desktop workloads were designed for local, dedicated disks that are generally much faster than users need them to be. With VDI, tens or hundreds of workloads access shared disks simultaneously and demand fluctuates throughout the work day. This makes it very difficult to guarantee user experience levels without over-provisioning storage capacity and throughput for the worst case.

The hypervisor complicates matters further. I/O from multiple workloads is consolidated and concentrated, which transforms potentially sequential I/O requests into random ones. Randomized I/O severely degrades the efficiency of traditional storage caching algorithms, leaving VDI performance highly sensitive to disk latency.

Variable and consolidated desktop workloads also give rise to intermittent load spikes. These I/O "storms" can occur when many users simultaneously log in, open the same application, run a virus scan, or perform some other common disk-intensive operation. I/O storms, when combined with the generally sporadic nature of desktop data access patterns, have stalled many VDI projects in the early proof-of-concept stage. And, In addition to requiring high levels of sustained aggregate I/O throughput, consolidated desktop workloads often exhibit wide variability in storage I/O read/write ratios.

In the end, delivering consistent and predictable performance for high-density VDI--at reasonable cost--demands innovation in both the hypervisor and storage layers, and deep integration between the two. Only a centralized storage infrastructure designed to handle these challenges without massive over-provisioning can hope to compete on cost against the current PC desktop status quo.

Next time: Storage innovators capitalize on VMware View's storage optimizations for higher-density VDI.

About the Author

A senior analyst and virtualization practice lead at Taneja Group, Dave Bartoletti advises clients on server, desktop and storage virtualization technologies, cloud computing strategies, and the automation of highly virtualized environments. He has served more than 20 years at several high-profile infrastructure software and financial services companies, and held senior technical positions at TIBCO Software, Fidelity Investments, Capco and IBM. Dave holds a BS in biomedical engineering from Boston University and an MS in electrical engineering and computer science from MIT.

Reader Comments:

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Sun, Apr 3, 2011 Ryan Snell Your back yard

"Only a centralized storage infrastructure designed to handle these challenges without massive over-provisioning can hope to compete on cost against the current PC desktop status quo." This describes the WhipTail Virtual Desktop XLR8r to a tee! The density problems, performance spikes, and overall VDI IO demand all center around the OS, not the data. When you add a WhipTail solid-state SAN/NAS appliance to the architecture to cover the OS then both the cost and performance barriers are shattered at once! In other words, move the OS write cache to a WhipTail appliance (View Composer, Citrix Provisioning Server, Microsoft Diff Disks, Unidesk, what-have-you) so you can scale that part of VDI out for less than $30 per user. Meanwhile, you keep all of the user data, profiles, etc. on your existing storage platform at the usual cost/GB. WhipTail can handle over 250,000 IOPS random write performance at 4k over 10 GbE, Fiber Channel, and/or Infiniband. The unit is also unified to support NFS and CIFS. That's more than enough performance to support THOUSANDS of users on a single appliance. (To be exact: 5,000 users at 50 IOPS per desktop on a single 2U appliance drawing less than 180 watts of power) The technology to solve this problem and restore order to the VDI TCO and ROI are available today. What are you all waiting for?

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