In-Band or Out?


Imagine driving a car on a stretch of highway. You have a clear view of the road ahead, but limited visibility of the overall network of roads, highways and byways surrounding it. That's the analogy InterSAN CEO Chris Melville uses to illustrate the drawbacks of using the in-band technique for storage management.

"Imagine you are an application with multiple paths to the data. You can only see one of those paths. You can't see what is above you and below you," Melville says. "You want a bird's-eye view. When you put the management in the data path, you have limited visibility. Management has to be where it effectively addresses the business need. Where it should not be is in the data path."

If the management is out of the data path,also known as the out-of-band or asymmetrical approach,then it has a helicopter-view of all the crisscrossed networks among the cluster of servers, switches and storage subsystems.

According to a January 2002 report issued by RBC Capital Markets, the contentious debate among SAN virtualization proponents includes where the intelligence of this architecture should reside. Some vendors support the in-band method (also known as the symmetrical or in-the-data-path approach), while others champion the out-of-band method.

DataCore Software, with its SANSymphony product, and FalconStor Software, with its IPStor software, favor the in-band method. Veteran storage players including IBM, with its StorageTank product, as well as Compaq Computer, with its VersaStor virtualization software, have been promising an out-of-band solution for months now. And both are late.

"That's the problem," says John Webster, an analyst at research firm Illuminata, based in Nashua, N.H. "Nobody has been able to get an out-of-band solution to market yet. Compaq's VersaStor was supposed to be that product, but that's a year-and-a-half late.

IBM's StorageTank was also expected to be on the market by now, but the company has pushed the product's debut back to later this year. "And that, I think, is more a wish than a reality," Webster says.

Now, IBM is talking about a hybrid, in-band and out-of-band virtualization product, Webster says. But, so far, Compaq continues to promise an out-of-band virtualization product, according to Mark Lewis, vice president and general manager of Compaq's enterprise storage unit.

During a public relations firm-sponsored dinner in Boston recently, Lewis publicly told a room full of storage vendors and journalists that VersaStor is officially in beta testing at one of Compaq's larger storage customers. The Houston-based company plans to start shipping the virtualization product sometime this year, he says.

In the meantime, RBC Capital Markets predicts the in-band and out-of-band debate will be a wash in the long run.

"Ultimately, we believe these approaches will converge in the marketplace as in-band intelligent aggregation agents collaborate with out-of-band metadata engines to deliver a single, coherent mapping of virtual volumes across the storage network infrastructure," its report states.

Still, others continue to take sides on which is the better approach. For instance, Gartner analyst Nick Allen favors the out-of-band virtualization method because you don't have to worry about running out of resources. The drawback with the in-band method is you can run into disruptive issues when a bus or disks run out of space and more resources either have to be added or found.

"We prefer the [out-of-band virtualization because it scales better. All that is happening with in-band is you are developing another controller, just like a disk controller," Allen says. "With out-of-band, I never run out of resources because I have not introduced a new box into the data path. With in-band, the question becomes 'How do I now introduce more resources in a nondisruptive fashion?'" n