VDI's storage hunger: Dollars per gigabyte and IOPS load don't mix

Don't underestimate the cost of storage for virtual desktop infrastructure. IOPS load can diminish some of the benefits of desktop virtualization.

Don Jones and Greg Shields, of Concentrated Technology, firmly believe that business comes first, not technology.

While they feel that virtual desktop infrastructure is undeniably cool, they don't necessarily believe that it offers true value to a lot of the businesses that pursue it. In this series of articles, the skeptics outline their arguments for and against various VDI technologies, helping you focus on what your business actually needs -- not necessarily on what a vendor or two might want to sell.


Have you ever tried to price out SAN storage without talking to a storage vendor's sales team? It's remarkably difficult. Enterprise-class vendors don't like to publicize cost figures. Storage area networks themselves are notoriously customized, with two SANs from the same vendor often having very different configurations and prices.

This said, storage area network (SAN) storage today can range from a few dollars to $50 per gigabyte and often more when redundancy, replication, disk-based backups and snapshots are factored into the calculation. Since every copy of your data adds to this number, costs skyrocket along with your data.

Add to this the data storage hunger of virtual desktop infrastructure (VDI). You'll quickly find that the dollars per gigabyte will be a primary driver in the overall cost of your virtualized infrastructure. For quick math, multiply the 10 GB to 20 GB of minimum storage space per virtual machine by the anticipated number of VMs, and you'll quickly find your VDI storage needs growing into the multiple-terabyte range.

More from the VDI Skeptics:

Four reasons why VDI might not be right for you

VDI isn't the only factor to consider for application delivery

Yet primary storage for VDI-hosted VMs is only the start. Add a percentage for growth along with another percentage for snapshots, plus a doubling or tripling of those numbers for replication and redundancy.

The density of that disk space will also create a major bottleneck, particularly in VDI environments where "heavy" applications are prevalent. In addition to architecting for raw disk space, you need to properly divide storage processing among available disks and their connected spindles. In short, too much disk activity centralized into too few disks creates a condition called spindle contention, where spindles lose performance because of excessive thrashing from too many requests.

In his article "Understanding how storage design has a big impact on your VDI," Ruben Spruijt explains how disk space for VDI environments is so much more than just raw storage. Also important is the input/output performance of that storage, "As it turns out," he writes, "there is a hidden danger to VDI. There's a killer named 'IOPS.'"

IOPS is the number of disk input/output operations per second in both read and write activities. Virtual desktops tend to be heavily write-oriented. Disk write operations are more costly than read operations and require more resources to complete. Early VDI prototyping may not take into account the extra load that VM processing places on your SAN storage. Test with simulations of your real-world workload to gauge how your infrastructure will actually work when loaded with users.

Planning a VDI deployment? These tips can help you succeed.

Learn how to assess your IT infrastructure for desktop virtualization

Read the reasons why virtual desktop projects fail

Check out Brian Madden's column on what not to do when virtualizing desktops

Calculate the return on investment from VDI

Spruijt suggests that virtual desktop architects need to plan for IOPS load to avoid poor performance after implementation. "It should be obvious by now that calculating the amount of storage needed in order to properly host VDI is not to be taken lightly. The main bottleneck at the moment is the IOPS," Spruijt writes. "The fact is that [VDI] demonstrates more writes than reads. And because writes are more costly than reads -- on any storage system -- the number of disks required increases accordingly."

Today's technology and skills might not be enough to reduce the effect of excessive IOPS. Rather, brute force may be the only acceptable solution. Faster and, more importantly, more disks may be necessary to deliver an acceptable virtual desktop experience to users. By spreading the VDI storage load across a greater number of disks or SAN frames, more spindles and associated storage processors can be brought to bear in fulfilling disk requests.

Although VDI promises benefits over a traditional desktop environment, these numbers point out that cost savings may not be one of them. Organizations with special desktop needs -- such as the ability to provision centralized desktops over long distances, support complex applications or rapidly rebuild labs -- may find the cost/benefit analysis in their favor. Those with more standard requirements, such as the typical office environment, could find excess cost and unnecessary complexity.

This was first published in September 2010

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