Calculating The True Cost Of VDI

In recent column, storage expert George Crump postulated that a VDI installation for an organization with 250 users could be done for less than $150 per desktop--not including whatever the user actually uses for hardware connecting to the VDI backend. I'd been doing some research on the subject, and was astonished at the $150-per-user price.

The problem, according to the story, is that storage is the bottleneck. More precisely, the bottleneck is storage bandwidth, not capacity. The assessment isn't all that surprising coming from a storage expert--when what you sell are hammers, every problem will call mostly for nails.

Before I get into offering my own price analysis, let me say that I agree with a number of points of George's article. First, DRAM is really important. If you want to see storage bandwidth become the bottleneck, just oversubscribe the server RAM, and--voila--you've got a storage bandwidth problem. Second, George says early on that the economics of VDI is a challenge. I agree with that, too. There are a lot of reasons to consider VDI or some other means of hosting apps that typically run on the desktop, but economics alone is not a good enough reason; the numbers just don't support it.

The first question you might ask is, $150 per desktop for what? We know the systems sitting on the end users' desks aren't part of that number, but certainly all the back-end software and hardware are, right? Probably not. The simplest way to purchase VDI is Citrix's VDI-in-a-Box product. It comes in 10 packs, with a list price of $1,950, which includes the first year of support. That's $160 for the license and $35 per year for the support. So much for a $150-per-desktop system. The VDI software will cost more than that.

Assuming you want to run Windows 7 Pro, you can tack on another $150 per user. And, no, you can't count the licenses you may have purchased with workstations. Microsoft will not allow you to transfer them or use them simultaneously. If the Redmond Mob audits you--and they're aggressive about doing so--you'll end up with a big fat bill for software.

You can already see why the economics of VDI are a tough sell. Assuming about $200 for the terminal devices on the users' desktops, we're already up to $545 in the first year, and we haven't even gotten ourselves any servers yet.

In terms of sizing servers, CPU performance is probably not going to be the limiting factor. If you bring up the Microsoft Task Manager on your own system, you'll see that CPU use rarely pops over about 20%. Accepting that at face value, you can figure five users per core, so a server with 16 cores should accommodate 80 users or so--provided you have enough memory. A typical Windows 7 desktop computer will come with 6GB to 8GB of DRAM. Mine has 6GB. I'm running Outlook, Firefox, Word and PowerPoint, and Task Manager tells me that 42% of my memory is in use.

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So, let's say we can get away with giving each user 4GB in a virtual machine on our VDI server. If we stick with the 80 users and don't oversubscribe the server's memory, we're looking at 320GB. That's a pretty hefty server. If we drop the memory allocation by half, we'll have a cheaper server, but we'll also ensure that we will be exercising that storage system and probably ensure that our users will notice pokey response.

The price on that four-socket, 320GB server will be about $20,000 or about $250 per user without any redundancy. If your philosophy leads you to N+1 configurations, you'd need four servers for 250 users or $320 per user.

Now the storage system. Obviously, there will a lot of duplicated files in a 250-user system. So, clearly, you don't need 120GB per user. Let's put the number at 20GB per user. That's a 5TB system you'll need, with some room to grow, so let's put the raw capacity at 10TB. Such a system will run you about $15,000, or another $60 per user for a system with limited performance and about double that for one with good performance.

Totaling it all up, we're looking at $380 to $440 for hardware costs, $345 for software and $200 for something on the user's desk. In round numbers, it totals $900 to $1,000, or about what it costs for a very nice desktop or a pretty good laptop.

The bottom line is you can justify VDI for a number of reasons--easier support, better security, perhaps better availability or a more appropriate system for task workers--but you can't justify it on hardware and software savings. The numbers won't work. If you have a large turnover in workers, or the need to frequently reconfigure desktops, or you have many remote workers, or you want to use VDI as part of a BYOD plan, then by all means take a close look. But do your homework on costs--they'll add up.