Solving the VDI storage problem

IT pros offer tips on building storage architectures for VDI and express opinions about the best storage products for virtual desktops.

Storage hungry virtual desktops drive up infrastructure costs, so a number of vendors have come up with money saving

ways to satiate the demand. The fact is, there is more than one way to attack this problem.

Some IT pros swear by the latest solid state storage technologies, and Citrix launched a promising cache storage technology in XenServer 5.6 last week for XenDesktop. Meanwhile, some IT pros say a traditional SAN works fine if it's built right.

Using Solid State Storage for VDI
Ken Adams, CIO at Miles & Stockbridge, a law firm based in Baltimore, has virtualized desktops using VMware View. He said he uses Dell Equallogic and an EMC Fibre Channel Storage array, and has put local storage on servers to improve performance. But it was not enough to support virtual desktops, particularly those that run video and dictation devices.

"I simply couldn't render desktops fast enough because of storage bottlenecks," Adams said.

He gave V3 Systems' Strato line of solid-state storage appliances a shot, and said "the performance was night and day."

V3 Systems' storage layer sits between the hypervisor and the hardware and re-maps the storage stack so that Windows desktops running above it take advantage of the solid state storage with minimal latency.

The Ohio Department of Developmental Disabilities also chose solid state storage for its VMware View virtual desktop pilot, with good results.

The DoDD's IT team did some dirty math to determine how much storage they would need to add to their Compellent SAN to support 1,400 virtual desktops. The answer was 300 to 500 hard drives.

The big problem is boot storms -- when everyone comes into work in the morning and signs in to their virtual desktops at the same time.

Todd Knapp,
CEO of Envision Technology Advisors

Instead, the team opted for WhipTail Tech's solid-state storage appliance. WhipTail says its XLR8r line solves the high I/O demands that processors and applications need for optimal performance by delivering 250,000 WRITE IOPS. 

"IOPS is critical to virtual desktop performance, and we knew we couldn't just throw spindles at the virtual desktop environment the way you normally would," said Kipp Bertke, IT infrastructure manager at the DoDD.

Using solid state means adding another layer of storage to manage, but the performance benefit outweighs the burden, Bertke said.

Other solid state options are available from Fusion-io, STEC and Xtremio. Another approach is to use storage virtualization to improve virtual desktop performance.

Citrix IntelliCache
Another option is local caching like that used in Citrix's IntelliCache storage technology. The technology appears in XenServer 5.6, which is supported by XenDesktop 5 Service Pack 1 (SP1).

This technology caches non-persistent desktop files on the local disk of the host server. By caching locally, a portion of runtime reads and writes of the virtual machine happen on lower-cost server-attached storage instead of NAS or SAN. Citrix claims this reduces storage requirements for central storage by as much as 90%.

The majority of enterprises run XenDesktop atop of VMware ESX though. So, unless those customers put XenDesktop on XenServer, storage costs remain an issue. But the IntelliCache technology probably won't be unique to Citrix for very long.

Dave Bartoletti, an analyst with The Teneja Group, a Hopkinton, Mass.-based IT analysis firm, said the local cache approach is an efficient, economical way to handle virtual desktop storage, and other vendors will probably develop similar technologies.

Using SAN for virtual desktops
While Citrix IntelliCache and solid state storage technologies can ease storage issues, there are ways to optimize an existing SAN for virtual desktops.

In fact, sometimes the problem is that the storage administrators simply don't know how to properly create storage architectures for virtual desktops, because the technologies are so new, said Todd Knapp, CEO of Envision Technology Advisors in Providence, R.I., a firm that specializes in building virtual server and desktop environments.

"The big problem is boot storms -- when everyone comes into work in the morning and signs in to their virtual desktops at the same time," he said.

Knapp said bottlenecks often originate in the I/O path, not the SAN. One way to alleviate performance problems is to use a NetApp performance acceleration card, which places a Flash cache between the workload and the storage array.  

One network engineer, Pacer Hibler, at New Hanover Regional Medical Center in North Carolina, uses XenDesktop with EMC Storage. To prevent storage overload, end users aren't allowed to store business data or personal configurations on VDI instances. Instead, employees save data on home directory and common files shares, which are backed up by EMC Networker. 

Most end users have a generic virtual desktop, because it costs less than personalized desktops. But when a personalized virtual desktop image is built for a specific end user, it gets exported to the SAN, which uses de-duplication techniques to compress the images to almost half their size.  

IT pros using a SAN for virtual desktop storage must be careful about overloading storage with high volumes of VMs though, which is a common mistake.

"I've seen people put 200 VMs on one LUN, and they are shocked when they have poor performance," Knapp said. "Well, you are burying yourself alive, so of course you aren't going to see good performance."

Another trick for eliminating storage bottlenecks is segregating swap file I/O from production I/O, and building dedicated storage for swap location to separate out the path to the hypervisor can eliminate the congestion, too, Knapp said.

"The future of virtual desktops is to have swap location and high I/O boot files occurring in solid state memory space," he said. "Whether it's done with direct attached, swap file path or solid state remains to be seen."

Let us know what you think about the story; email Bridget Botelho or follow @BridgetBotelho on Twitter.

This was first published in May 2011

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