In-Band or Out-of-Band, That Is the Question

"Imagine you are an application with multiple paths to data. You can only see one of those paths. You can't see what is above you and below you," says Melville. "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. What it should not be is in the data path (In-Band)."

If the management is out of the data path -- also known as Out-of-Band -- 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 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 way. Both have products on the market. But veteran storage players like 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, NH, "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 a half late.

"We were supposed to have seen IBM's StorageTank," he says, "but now they have pushed that back to later this year. That, I think it's more wish than a reality."

Now, IBM is talking about a hybrid, In Band and Out-of-Band virtualization product, says Webster. 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. More recently, Lewis publicly told a room full of storage vendors and journalists that VersaStor officially is in beta testing at one of Compaq's larger storage customers. The Houston-company plans to start shipping VersaStor sometime this year.

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 agents aggregation agents collaborate with Out-of-Band metadata engines to deliver a single coherent mapping of virtual volumes across the storage network infrastructure," according to the January 2002 report.

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. "We prefer the asymetrical (Out-of-Band) virtutualization because it scales better. All that is happening with In-Band is you are developing another controller, just like a disk controller. 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 introduce more resources in a non-disruptive fashion?'"