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How to get the most out of VDI IOPS

A lot of factors affect how your VDI storage system uses IOPS. Consider the master image, OS features and more.

You've heard it before: VDI is all about the storage. But have you ever considered how the virtual desktop master images factor in?

When it comes to virtual desktop storage planning, it's easy to focus solely on the storage hardware architecture. But there are some unusual factors that should be on your radar: the master image, for instance, and features in your operating system. All of these can affect how the storage system uses IOPS.

It's important for virtual desktop infrastructure (VDI) storage to use IOPS in the most efficient way to optimize virtual desktop performance. How many IOPS do your users and their applications require to run efficiently? Can your existing storage array support that amount of IOPS or do you need a new one?

To determine the VDI IOPS you need in your environment, there is no substitute for benchmark testing. Still, it is possible to roughly estimate a host's I/O requirements. An optimized Windows 7 desktop generally produces about 10 IOPS (Windows XP produces about 8 IOPS). Of course these estimates vary greatly depending on the workload that the users are generating and on the applications they're running.

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Also look at your OS: Windows 7 and all previous Windows desktop operating systems were largely designed under the assumption that the operating system would be run on physical hardware. A single Serial Advanced Technology Attachment (SATA) disk in a PC can usually deliver 40 to 50 IOPS (depending on a number of factors). That being the case, the Windows operating system was not designed to minimize IOPS.

That isn't to say that Windows 7 and some of the previous Windows OSes don't contain disk-optimization mechanisms -- they do. In a VDI environment, however, such mechanisms can do more harm than good, which makes it critical for you to decide up front which operating system features should be disabled. Then, you can begin benchmark testing to determine the overall performance requirement for your storage subsystem.

What OS features do you need?

Administrators will need to decide which OS features their organization does and does not need. As a general rule, disk defragmentation should be disabled. You can and should defragment the master image as part of the creation process, but afterwards, disable the defragmentation feature before you create any virtual desktops.

It is also a good idea to disable the Windows prefetching feature. Prefetching places commonly read files into a special cache that is designed to speed up file reads. In a VDI environment, however, the prefetching process increases I/O on the physical storage without delivering any real benefit to the virtual desktops.

All this makes a virtual desktop storage project a complex effort. Still, if built right, shared storage provides an effective tool that's able to withstand a host failure. So decide what it is you want from VDI storage, how much you're able to spend and what level of VDI IOPS performance your organization demands.

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