Citrix Systems has developed a new caching feature that may ultimately address nagging I/O challenges for server workloads.
Designed for virtual desktop infrastructure, Citrix Systems' Inc.'s IntelliCache can reduce I/O over the storage network between servers hosting virtual desktop and a back-end network-attached storage (NAS) array where system images are stored. It works somewhat like wide area network optimization technology by deduplicating and caching redundant bits and is deployed as part of the hypervisor. But the feature needs further development before it's ready for prime time in server virtualization environments.
In VDI environments, a user boots a desktop by pulling a master image from the back-end NAS to a local server, said John Humphreys, Citrix's senior director of product marketing. IntelliCache then "watches" data go back and forth from the master machine image on the NAS filer to the local server hosting the virtual desktop. When additional users boot up virtual desktops, IntelliCache ensures that locally cached blocks of data are read first, rather than fetching them from the back-end array.
Getting control of virtual servers' storage I/O
Citrix isn't alone in trying to tame storage I/O in virtual server environments. This year, VMware introduced Storage I/O Control, a kind of quality-of-service measure for vSphere administrators using networked storage devices. Similarly, IntelliCache has the potential to reduce server I/O constraints for users such as cloud providers with a fixed menu of OS images, or applications such as Microsoft SQL Server and Exchange, Humphreys said.
Other use cases could emerge, said Gartner analyst Chris Wolf, such as training labs at trade shows and universities where servers boot from a master image, or in test and development environments. He agrees that cloud computing would also be a likely niche, given the general preference among service providers for farms of servers with local storage.
At least one XenServer user understands IntelliCache's potential on the server side. "The Master Image concept and less I/O for XenServer VMs would be welcome, especially for those remote users that may have low bandwidth availability," wrote Larry Dajnowski, an MIS manager at Himmel Management Co. LLC in an email. "[It could] also simplify the complex XenServer environments that exist today since Master Images could be used for multiple VMs." Dajnowski is currently testing the Cowley release, including IntelliCache for VDI.
IntelliCache not ready for server virtualization prime time
But IntelliCache isn't there yet. "Future enhancements would be necessary before a [virtual server] rollout would be done using this IntelliCache technology," Dajnowksi said. Indeed, an enterprise virtual server version of IntelliCache would require a "second to third generation improvement in technology," Citrix's Humphreys conceded.
But several issues still impede the use of IntelliCache in server environments. Today, for example, it can technically cache any Linux-based virtual machine, but IntelliCache version 1 has no way to save changes to the master images that are made during a user session, though master images can be modified on the administrative end.
For this reason, it's doubtful a server-side IntelliCache would find a use case in his environment, said Phil DeMeyer, an information technologist at an early-childhood education program in the Midwest. "Any savings you would see from provisioning one golden image, you'd have to make up for in managing delta files for applications like Exchange and SQL," he said. "I can see it for XenApp, but you'd have to have a pretty big Citrix farm to make it worthwhile."
Another feature that would need to be developed for IntelliCache to be relevant for enterprise virtual servers is the ability to perform live migrations of machines during local writes. And should IntelliCache enhance its ability to manage master image changes, it's unclear how much memory and/or local solid-state disk drives would be needed in the server to accomplish this with effective performance.
That's the real kicker, said Gartner's Wolf. For enterprise servers, "without live migration, [IntelliCache] is impractical today."
Beth Pariseau is a senior news writer for SearchServerVirtualization.com. Write to her at firstname.lastname@example.org.
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