IP and Fibre Channel create flexibility

Rival technologies can co-exist, says Alan Stevens.

Storage area networks (Sans) are becoming increasingly popular, primarily because they enable firms to configure and manage storage independently of the servers that access it.

Sans also separate storage traffic from the main local area network (Lan), freeing bandwidth, and allowing data to be located at long distances from client servers - up to 20km or more with the latest fibre technologies.

Such benefits are proving increasingly attractive to corporates, although interoperability problems and the high cost of implementation have deterred some buyers. Even so, in a survey, some 30 per cent of companies in the UK said they had implemented a San of some kind and, despite the recent economic downturn, the leading storage vendors predict this figure to rise by between 70 and 80 per cent by 2003.

Two competing types of San architecture are in evidence. One camp favours Fibre Channel, the technology on which virtually all current San solutions are based. In the other camp are those who want Sans based on IP, the protocol that underpins the internet and most corporate Lans.

The two approaches are quite different. Fibre Channel is a very mature technology, and one that provides a proven robust, storage-centric infrastructure.

Fibre Channel products are widely available from a range of vendors, most of which have either announced or are already shipping the latest 2Gbps switches and host bus adapters (HBAs). These offer double the bandwidth of early 1Gbps Fibre Channel products, and 10Gbps devices are also being prepared.

Interoperability issues

Interoperability among Fibre Channel products is much less of a problem today, thanks to the latest standards. Also, the leading San management software products are all geared to work with Fibre Channel.

IP-based San systems such as iSCSI have only recently begun to appear.

Only in the past 18 months have there been any serious attempts to overcome difficulties associated with distance and latency to allow IP to support San applications.

Although the first products to use the new iSCSI protocol, on which IP-based solutions are based, have been announced, they are not expected to ship until later this year at the earliest. Even so, the IP lobby has gained considerable support recently and its momentum continues to grow.

Among the companies promoting iSCSI are Adaptec, Hewlett Packard (HP), IBM and Qlogic, all of which have announced products. In addition, most of the developers of San management tools have announced that they will support the protocol. Datacore is the latest to announce iSCSI support for its popular SanSymphony suite.

But can iSCSI ever evolve to threaten the dominance of Fibre Channel? Paul Trowbridge, marketing director at San fabric switch vendor Brocade, does not think so. "Brocade doesn't really see that happening: iSCSI is still very much in the hype phase today with a lot of benefits being cited, most of which are myths," he said.

Some argue that because Ethernet is migrating to 10Gbps, an IP San will be faster than Fibre Channel, but Trowbridge believes they are mistaken, especially as 2Gbps Fibre Channel products that offer superior performance are now starting to ship.

It is also debatable whether low-cost Ethernet NICs could be used to build IP-based Sans. For iSCSI to work efficiently, application-specific Ethernet interface cards are required to offload the running of the IP stack and the iSCSI protocol. These will cost about the same as a Fibre Channel host bus adapter (HBA).

Another belief is that existing Ethernet infrastructure could be used. This does not take into account the very specific protocol requirements associated with SCSI and logical unit number level masking used to present the storage to the server.

New switching products will be needed to support these and other San services, such as the Distributed Name Service, and additional storage management intelligence will be required.

Trowbridge believes that business considerations will also promote Fibre Channel Sans. "Customers want to build their storage infrastructures now and simply can't wait the two to three years we believe it will take to sort the iSCSI standard out," he said. Brocade believes that IP will have an important role to play in storage networking but not at the expense of Fibre Channel. It believes corporate Sans will use multiple protocols.

"Fibre Channel will continue at the core as it will still be the most efficient and most reliable technology for block I/O data movement within a data centre," said Trowbridge. "The Fibre Channel IP protocol being developed with Cisco will be used to link data centres over IP, with iSCSI used for remote San access from smaller branch offices."

HP looks set to be one of the first server vendors to ship iSCSI-based San solutions. It is working on solutions with Adaptec and said it will ship products before the end of the year.

Brice Clark, director of strategic planning for HP's network infrastructure solutions, pointed out that Fibre Channel Sans are deployed primarily in data centres to address the need for highly scalable storage, servicing server farms and large database applications. "The catch is that Fibre Channel is another network and it requires another set of expertise," he said.

SCSI expertise

This is where iSCSI - an extension of SCSI - has a key advantage. The iSCSI protocol encapsulates SCSI commands in TCP/IP, enabling Gigabit Ethernet - and in the future 10 Gigabit Ethernet - to be used as a physical transport for Sans. The advantage over Fibre Channel is that most firms already have considerable knowledge of Ethernet and TCP/IP technology on their networks, but are relative strangers to Fibre Channel.

Clark acknowledged that iSCSI may not yet be ready to affect San sales, however. "While Ethernet and TCP/IP are clearly mature and exhibit high interoperability among vendors, iSCSI is new and still under development and there are still issues to work out," he said.

Most servers implement TCP/IP in software, for instance. This means that for iSCSI to support a primary storage network that offers comparable performance to a Fibre Channel architecture, the iSCSI protocol and TCP/IP processing will need to be offloaded to high-performance Gigabit Ethernet HBAs similar to the Fibre Channel HBAs currently in use.

Many suppliers have iSCSI products under development, and the first shipments are expected late this year. Storage products such as disk arrays and tape libraries with iSCSI and Gigabit Ethernet interfaces will also become available towards the end of this year, and some pre-standard products are already shipping. It seems likely that the San market will see an explosion of iSCSI products and solutions throughout 2002.

Like Trowbridge, Clark thinks that iSCSI will not replace Fibre Channel, and that the latter will continue to dominate San architecture, although for different reasons.

"Fibre Channel will continue to grow and be the preferred choice of storage experts who know how to deploy it. But iSCSI will appeal to server and network experts who know Ethernet and TCP/IP. Another way to look at it is that iSCSI will grow the whole storage networking pie by bringing the benefits of pooled networked storage to new customers in a more familiar way," he argued.

There is wide agreement in the storage industry that there is room for both iSCSI and Fibre Channel Sans, although there are differences of opinion when it comes to timeframes and the relative merits of the two technologies.

What is interesting is that neither camp expects firms to give up existing San architecture and investments to rush headlong to adopt IP solutions.

Infiniband Sans

In addition to Fibre Channel and iSCSI, the San market may see a third competing technology emerge in the form of InfiniBand, Intel's new input/output (I/O) architecture.

InfiniBand was designed as a replacement for the PCI expansion bus, but could replace Fibre Channel connections over short distances between servers and storage devices in densely packed data centres and server farms. The first incarnation of InfiniBand offers bandwidths up to 2.5Gbps, but higher-speed 10Gbps components are on the roadmap.

Brocade has already announced forthcoming products that will support InfiniBand, while Intel is in a prime position to encourage widespread adoption of InfiniBand technology by integrating it into server motherboards.

It is not clear whether each InfiniBand device will need a separate IP address, however, which may deter firms from adopting it until IPv6 is more widely deployed.