IP storage is growing fast, even if it's not evolving in quite the direction that was predicted for it. So says SAN guru Tom Clark, who admits that he and other proponents of IP storage may have overstated the likelihood of it replacing today's Fibre Channel SANs wholesale. Clark should know what he’s talking about – he is author of one of the definitive texts in this field, Designing Storage area Networks as well as IP SANs.

Far from supplanting existing SANs, IP storage technology is instead being used to extend and interlink them. It is also bringing the advantages of SANs to a whole raft of smaller servers, where Fibre Channel adapters would be uneconomic.

Fortunately, Clark is sanguine about this mis-estimation, not least because he works as director of strategic marketing for Nishan Systems, whose technology bridges the IP and Fibre Channel worlds, and which has now repositioned itself to take advantage of the convergence between the two.

"There has been a fairly radical change in our message to the market," he says. "The original message was 'Death to Fibre Channel' but the new message is based on what we found in the market - that the value is in internetworking. Our expertise is in marrying technologies, so we hope the transition period gives us a good life."

The big name is IP storage is of course iSCSI, which allows block data to be sent over an IP network, effectively creating an IP SAN. This was ratified earlier this year after more than two years of development work; some have described this as slow progress, but Clark says they misunderstand the amount of work involved.

"There was a lot of hype about iSCSI, but I thought it went fairly quickly - it's mindboggling the amount of work that goes into these projects," he says.

Because many IP storage supporters originally positioned iSCSI as a direct rival to Fibre Channel, existing SAN vendors have criticised it for its inability to provide equivalent performance without an additional special-purpose network adapter.

Called a TOE, for TCP/IP Offload Engine, these adapters have their own processors so they can relieve the main CPU of the extra work involved in handling iSCSI packets. The problem is that adding a TOE is not much cheaper than adding a Fibre Channel adapter, which would seem to negate some of iSCSI's cost advantages.

However, Clark says that many servers have spare compute power, and not all applications need that much speed anyway. "Fibre Channel still has the advantage for high performance storage access, but the key is the ability to bring in a whole class of servers that don't need Fibre Channel or Gigabit performance," he says.

"Four-CPU servers have plenty of cycles left, so why not use those? For example, we're getting 30MB/s writes and 50MB/s reads via a standard Gigabit Ethernet card, at 17 percent utilisation on a dual-processor server.

Clark argues that IP storage is preferable, not just because it is cheaper, but because it is the most popular technology: "Ultimately, everything becomes a native routable IP format. IP has a performance overhead, but it doesn't matter anymore - we have silicon to deal with the overhead."

He adds that iSCSI represents the convergence of SANs with other networks, and that from now on storage is 'just another protocol'.

"The main kicker is Microsoft," he says. "By blessing iSCSI and including it in Windows 2003 with the iSNS storage discovery mechanism, it is stirring customer interest, whether they are existing Fibre Channel customers or not."

And with modular storage arrays, it means that companies can build scalable infrastructures and gain the advantages of a SAN, without needing to buy Fibre Channel adapters and get into fabric management.

Nishan's role is to link Fibre Channel SANs to IP networks, letting remote SANs share assets, for data replication say, and giving IP hosts access to Fibre Channel resources. Its iFCP protocol allows the administrator to 'export' resources from one SAN to another; the SANs themselves are not combined, so namespace scalability and SAN reconfiguration problems are avoided.

Its boxes combine the functionality of a Fibre-IP bridge with that of a Fibre Channel switch, so they can even be used to create an all-in-one SAN for branch sites. "You can have Fibre Channel devices on an IP backbone - you still have to understand storage networking, but you don't need that unique alien architecture, the Fibre Channel fabric," Clark says.

On the other hand, he acknowledges that Nishan has no interest in Fibre Channel going away too quickly, because its competitive edge is its ability to interconnect those different networks. "We would like it to be in co-existence for as long as possible," he says, "because when it all goes iSCSI, we'll have to find another trick."