Virtual Desktop Storage Basics: Understanding storage requirements

In the second segment of this four-part e-book, we’ll help you understand your virtual desktop storage requirements and manage storage for optimal performance.

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Virtual desktops may make management easier, but proper planning is needed to reduce storage bottlenecks, ensure performance and accommodate growth. Storage subsystems can ease VDI deployments, whose costs can balloon if you don't follow best practices for supervision. In this second segment of a four-part e-book, we'll help you understand your virtual desktop storage requirements for optimal performance.

Storage is a critical aspect of desktop virtualization. The client/server model that provides application and desktop instances to end users also relies heavily on network-based shared storage. But the demand for storage poses serious problems for IT administrators. Storage systems must be designed for adequate performance under random I/O loads. Providing sufficient storage capacity for virtual instances and backups can stretch lean capital budgets to the breaking point. Administrators can save money and often improve storage performance by understanding the storage needs of desktop virtualization deployments.

Estimating storage for desktop virtualization
An organization needs to allocate the right amount of storage for a virtual desktop deployment. For example, too little storage capacity or performance will impair the performance of virtual instances, while too much capacity wastes capital. Unfortunately, determining the appropriate amount of storage can be much harder than it appears. There is no single universal formula or tool that will delineate storage needs for you -- it's part science and part art.

Invest the time and effort to understand the storage needs of desktop users. Start with performance rather than capacity. "It comes down to measuring the desktops and then figuring out the current IOPS per desktop and the profile of that usage, reads versus writes, sequential versus random access," said Keith Norbie, vice president of sales at Nexus Information Systems in Minnetonka, Minn.

Software tools can help you identify these performance characteristics. Adding the performance needs for each user can build a picture of the total, cumulative performance required from the storage system. And any evaluations should reflect different times of day, different times of the month, and different users or user groups.

Design the storage system to exceed those performance levels wherever possible because that will affect scalability. Admins often don’t account for the increased load of more users in virtual desktop deployments. "I've got a marketing company in the Twin Cities that approached VDI with sort of a 'try it' attitude," Norbie said, recounting a limited deployment with a small Hewlett-Packard storage array where about 14 spindles supported 300 to 400 virtual desktop users. It did not take long for performance problems to become evident to users. "You took 400 I/O generators [individual PC disks] out and replaced them with 14," he said.

Figuring the actual storage capacity requirements can be equally challenging. Virtual desktops are usually deployed based on a very limited number of standardized "golden images." For example, 100 users may use one 50 GB golden image, 600 users may use a second 50 GB golden image, and 300 users may run a third golden image. Each golden image would be modified by each user’s profile to provide a modicum of personalization to the otherwise generic desktop. This means 1,000 users can be serviced by about 150 GB to 200 GB of enterprise-class storage for the golden images and user profiles. If the same 1,000 users were each given 50 GB of storage for unique desktops, an organization would need to add an astounding 50 TB of enterprise-class storage.

Don't forget user data and backups. Even though applications like Word or Excel might be part of the same golden image, the documents and spreadsheets created by each user need space to reside. A good ballpark estimate is to take the approximate user data space in a cross-section of individual PCs and then multiply that average by the total number of users.

For example, if an average desktop user needs about 20 GB for data, set aside 20 TB or more for 1,000 users. While desktop images are typically loaded from a SAN, user data is frequently redirected to network-attached storage (NAS). It's much easier to create folders for 1,000 users' data rather than to create and maintain 1,000 logical unit numbers (LUNs) on a SAN, greatly simplifying administration. In addition, data can be stored on larger, less-expensive disks.

Managing storage needs for virtual desktops
Virtual desktop deployments almost always grow as organizations gain experience with a technology and new use cases are identified. The problem is that storage demands and network traffic multiply as virtual desktops proliferate. This makes monitoring and capacity planning particularly important.

"Have that visibility down into the storage system to understand capacity utilization, performance, potential bottlenecks, data protection and even DR [disaster recovery] scenarios," said Mark Bowker, an analyst at Enterprise Strategy Group in Milford, Mass. "And where administrators can also provision new desktops, upgrade and patch the desktop environment."

The idea is to track growth data and then correlate that data against business needs to determine how storage -- as well as network and server -- needs are changing. For example, adopting a new application for a broad base of virtual desktop users may require a new golden image, along with significantly more storage for each user's data. Having the additional resources in advance can prevent performance and availability problems that would harm user productivity.

The combination of desktop golden images, user data stores and backup space can add up to a significant amount of storage that ultimately costs money to buy and manage over time. Several tactics and technologies can help you reduce storage requirements and improve performance. First, desktop virtualization often compromises the versatility of stand-alone PCs for the security and manageability of virtual instances. But this compromise isn't always suited to every user in every organization. Certain user groups may benefit greatly from desktop virtualization, while other groups or individual users may need unique desktops. Eliminating unnecessary users from the virtualization project's scope can reduce demands for storage.

Also take steps to reduce the footprint of virtual desktop images. For example, removing unnecessary Windows components and limiting applications can dramatically shrink the golden image. This results in faster load times, faster backups and lower backup storage needs. Similarly, adjust backup schemes to reflect the needs of each user group. Agents on the sales floor may need frequent backups, while back-office employees running different applications in a different golden image may need far less frequent backups of their user data.

Consider storage systems with storage-reduction technologies such as compression and data deduplication. Both technologies remove redundant data from storage and reduce capacity demands, reducing capital storage investments over time. Virtualization planners will need to know more about the storage architecture, so testing data-reduction technologies early in the proof-of-principle phase can help determine total storage needs.

Finally, take steps to reduce or avoid disk-intensive user activity that can severely hinder storage performance across large virtual desktop deployments. A prime source of trouble is antivirus software. "If you have a bunch of virtual desktops all performing antivirus scans at the same time, that's not preferred for large-scale VDI environments," Bowker said. "That's why we’re seeing the vendor community look at other ways to offload that antivirus scanning and even offload that into the available resources of the storage system."

In the third segment of this four-part e-book, we'll help you understand storage allocation for virtual desktop environments.

This was first published in July 2011

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