Visions of virtualization

Server- and array-based virtualization, in which the software and data-pooling intelligence reside on the server or storage array, were the first virtualization attempts. Vendors with a proprietary interest in servers or storage arrays manufactured virtualization software that ran on each of these devices.

For example, EMC virtualizes data across the drives of individual Symmetrix arrays.

Because array- or server-based virtualization doesn't put additional devices in the direct path of the data, it scales better than network-based virtualization, says Jamie Gruener, an analyst with The Yankee Group.

And deploying virtualization on the host server doesn't burden the other devices in the network, such as Fibre Channel switches or storage arrays. Of course, this method puts an extra burden on the server that has to process the extra virtualization tasks, so it could cause server-based latency.

Vendors argue that array-based virtualization lets them fine-tune all the virtualization capabilities because they are already familiar with the inner workings of the array.

But Steve Duplessie, an analyst with Enterprise Storage Group, says he "doesn't see any advantages [to array-based virtualization] other than we expect RAID boxes to be smart appliances, and that should continue."

By far the most vendors seem to be lining up behind in-band virtualization software that sits on an industry-standard Intel server running Windows NT/2000 or Linux.

Network-based virtualization suffers from the same latency problem that server-based virtualization does: It puts a burden on the other network servers to always have to look to the virtualization server for information on where their data is. And it exacts a certain performance penalty on the server performing the virtualization.

Analysts also have concerns about the hardiness of the server deployed in network-based virtualization. "The knock against current in-band [virtualization] methodologies is that at some point they will become a bottleneck because they reside on Intel servers," Duplessie says.

He adds that while vendors such as DataCore and FalconStor take various approaches such as caching to mitigate potential latency problems, the point remains that the server the virtualization software is deployed on is often less powerful than the servers managing data and I/O.

"At the end of the day, there is an NT box [whose bus structure is not optimized for I/O performance] in the middle of the road," Duplessie says. "Large companies are very leery about NT being the core of their enterprise virtualization infrastructure."

If latency is a concern, Wayne Lam, vice president of engineering for FalconStor, suggests users try his company's IPStor product, which runs on Sun Solaris as well as Intel servers.

Another concern is that the typical Intel-based server's I/O is not particularly well-suited to configurations where snapshot backups, data replication or caching take place, analysts say.

Compaq and Sun say their upcoming virtualization technologies will overcome these issues. The two companies will introduce hybrid network virtualization devices that exist outside the data path and do not affect the transfer of data between host servers and storage devices.

Duplessie agrees that out-of-band virtualization schemes scale better than in-band. "There's never a scale problem; therefore large distributed enterprises have less of an issue with this approach. However, the downside is putting software on each and every host in the storage network," he says.

An advantage of in-band appliance-oriented virtualization is just that- users need no code on the host servers and because every I/O request and response passes through the virtualization engine, nothing else is required. Appliance-oriented installations are simple and easy to maintain, analysts say.

What's next: specialized virtualization switches.

Ask analysts where they think virtualization needs to go to benefit users, and they express a common theme.

"The concept of storage abstraction, or virtualization, is here to stay and that users will find more benefit than problems in today's world," Duplessie says.

"While the choice you make today may not ultimately be the best one, you have to start somewhere," he says. "The ability to manage disparate devices under a unified virtualization schema outweighs the fact that you may move to a different architecture down the road."

Duplessie looks forward to a new brand of purpose-built switches from vendors such as Pirus Networks and Maranti Networks that feature virtualization running at wire speed with no latency.

And Gruener says users considering virtualization software shouldn't consider any scheme that doesn't support a high-availability configuration or additional components that protect the storage network.

"On top of it you will have all sorts of services that are woven into the virtualization tool set- mirroring, capacity on demand, snapshot backup, data replication," Gruener says.

"The biggest challenge right now is that virtualization as a whole isn't necessarily going to be a separate feature longer term. It's going to be a component of a larger management package," he adds.

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