Guide to calculating ROI from VDI
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[This feature was updated June 2013]
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Whether or not a virtual desktop infrastructure (VDI) requires a SAN is open to debate, but experts and administrators agree that if you do opt for a SAN for VDI storage, you’ll need to do a good deal of research and planning to find the right balance between performance and VDI costs.
A VDI environment provides several advantages over traditional PC-based workstations, including better security and simplified management and provisioning, but storage performance issues and costs have made it less appealing for many. To provide end users with an experience equal to or better than the PC desktop, administrators often have to use large amounts of traditional spinning disks or employ expensive solid-state drives (SSDs). They risk either spending so much money on performance that the project doesn’t make sense, or sacrificing performance to keep VDI costs down.
After speaking with industry experts and an IT administrator, we have put together advice for making sure your SAN performs well enough to support a VDI environment without spending more than you have to.
Virtual desktops explained
Desktop virtualization stores a user’s desktop as a virtual machine (VM) on a central server. Users can access their desktops from anywhere through a thin client, which can be low-cost hardware, a personal computer or an application installed on any PC. The desktop is hosted on servers that handle the desktop processing and applications. “You are freed from the hardware management,” said Sam Lee, a VDI deployment veteran and the senior solutions architect for Force 3, a Crofton, Md., solutions provider. “It’s a complete transformation in terms of IT management.”
All VDIs are managed at the data center instead of individually on users’ PCs. Updates, patches and configuration changes are made on the centrally hosted and managed VMs.
VDIs also offer better security because everything is stored in the data center as opposed to users’ hard drives, which can easily be taken out of an office or get hacked. A VDI also helps data recovery because the thin clients don’t hold any storage. If they fail, users’ data is in the data center, not stranded on a failed disk in the field or a remote office.
The dominant virtual desktop applications are VMware Inc.’s VMware View and Citrix Systems Inc.’s XenDesktop. View runs on VMware’s vSphere hypervisor platform. XenDesktop can run on Citrix’s server virtualization platform, XenServer, vSphere or Microsoft’s Windows Server 2012 Hyper-V platform. Microsoft also offers a virtual desktop product called Microsoft Windows Server 2012 Remote Desktop Services, and Dell Inc. has its own VDI offering called vWorkspace.
Planning your VDI deployment
Despite the advantages, it can be a frustrating balancing act between giving users the same or better performance experiences than their traditional PCs and keeping infrastructure costs in line. “You have to provide two things,” Force 3’s Lee said. “The first is that [virtual desktop performance] has to be as good as the existing physical desktop. Otherwise, the users won’t accept it. Second, the cost of VDI has to be comparable to a desktop.”
VDI environments have different I/O requirements from typical desktop PC environments. Because of the random nature of I/O requests from hundreds or thousands of desktops constantly bombarding the storage infrastructure, typical caching won’t help. Plus, virtual desktops have different read/write requirements than desktop PCs, which typically do more writes than reads at boot but handle more reads than writes the rest of the day. A VDI environment performs more writes than reads throughout the day, putting more strain on its storage infrastructure.
From his experience implementing VDIs for dozens of customers, Lee recommends using SSDs for I/O-sucking operating system boots and using a traditional SAN with cheaper SATA drives for user data. He sets aside 3 GB per user on an SSD drive for I/O and 10 GB per user for storing user data.
You also have to look at the operating systems you will use, said Seth Knox, director of marketing for Atlantis Computing, which sells virtual appliances that optimize storage for VDI. For example, using Windows 7 instead of Windows XP will double your IOPS needs, Knox said.
Determining how many IOPS you’ll need per user is the difficult part. “Often, people undersize it and use between 4 and 12 IOPS per desktop, when they really need three, four or five times more than that to get decent performance,” Knox said.
Storage vendors, hypervisor vendors and VDI providers all have their own recommendations, and they can be all over the map.
During his VDI project planning, Brian Brothers, network administrator manager for Ohio’s Department of Developmental Disabilities (DoDD), consulted with storage vendors, VMware and other administrators with VDI deployments. But they all gave him different IOPS requirements. “The numbers we got varied anywhere from 5 IOPS per machine to up to 40 IOPS per machine,” Brothers said.
Russ Fellows, a senior partner with the Evaluator Group, an IT analyst group based in Boulder, Colo., said he found the same thing when he researched VDI IOPS requirements for enterprise clients. “That’s when we said we need to come up with something that allows users to accurately measure and compare storage systems against each other,” Fellows said.
So the Evaluator Group published a VDI storage benchmark, the VDI-IOmark. The benchmark is available to users and vendors. Fellows said users run the benchmark to get a better idea of how much the storage portion of a VDI implementation would cost and to compare storage systems VDI costs.
You have to consider the average number of IOPS a user will need and also account for peak periods, such as at the beginning of each day, when everyone boots their thin clients and logs in to the network. “Peak processing is very important because if you get bogged down during the peak time, everybody is waiting to get things to work,” Lee said.
While doing his own benchmark to repair a VDI deployment that went awry, Lee said he found that a typical Windows desktop uses 50 to 80 IOPS on average, including boot storms.
Atlantis Computing’s Knox said the minimum amount of IOPS that anybody should plan for is 30 IOPS per user. To make the virtual desktop experience at least as good as a physical desktop experience, he recommends at least 50 IOPS per user.
After you’ve determined your performance needs, look at capacity. There are two major considerations for determining VDI storage capacity: whether the desktops will be persistent or stateless and how the system is configured.
Persistent vs. stateless desktops
A persistent desktop contains everything the user needs within the virtual desktop image and the individual VM. All of the applications, documents, settings and profiles are tied to the individual desktop and VM. A stateless desktop uses one master image for all users (also referred to as a “golden image”), and the profiles, settings and applications are streamed to the thin client, much like Netflix streams movies to your television. “That’s an important distinction because it really affects the cost of storage,” Knox said.
A stateless environment uses a single image for all virtual desktops, eliminating the need to store individual images for each user. Instead of updating each virtual desktop individually, an update or patch to the one image updates every virtual desktop.
If you’re not equipped for a stateless VDI, you can begin your deployment with a persistent setup and migrate to a stateless environment later. “A lot of people want to get to the stateless environment but have constraints where they have to deploy persistent first because it’s going to take time to migrate all of their desktops, virtualize all the applications and implement profile virtualization,” Knox said.
How you configure your VDI environment and whether you use data reduction technologies can have a big impact on VDI costs. “Proper configuration for VDI is probably more important than any other application, and it has a huge impact on cost, performance, capacity, all of those things,” Fellows said.
One option is using stateless desktops with a master image that links to all desktops instead of deploying unique images to each desktop. Another option is using data deduplication, because even if you use unique images for each desktop, a high percentage of each image will be the same.
Fellows said proper planning and management can make your storage costs for a VDI environment reasonable. “If you configure things really intelligently, you actually can get your costs down to under $100 per desktop just for the storage,” he said. “If you don’t, your cost with the exact same gear could easily be 10 times that.”