A technology that promises to bridge the worlds of data storage and networking has passed a key point on its path to becoming a usable standard.
The iSCSI (Internet Small Computer System Interface) specification has been recently finalised by the Storage Networking Industry Association (SNIA), and the Internet Engineering Task Force (IETF) is expected to endorse the standard with minimal changes. According to Steven Bragg, product marketing specialist at StorageTek, the leading vendors were already so close to the standard that companies such as Cisco were already selling compatible products, while others were on the brink of releasing them. "I believe iSCSI solutions are all but here today," he said in mid-September.
EMC product manager Abie Gelbart says "it's still a little premature to get too excited" about iSCSI, but adds that the technology will be an important part of storage in the next few years. The company is "very positive about iSCSI," he says, and it was heavily involved in the development of the standard.
iSCSI extends the reach of SCSI storage devices from a few metres to large distances by transporting the commands and data across TCP/IP networks. Affordable gigabit Ethernet components mean the idea of running block-mode disk traffic over TCP/IP LANs is feasible.
The requesting computer encapsulates SCSI commands and data in TCP/IP packets, which are transmitted across the network to the storage controller. The commands and data are extracted and passed to the storage device, which carries out the appropriate operation. Any data returned by the device (eg, the data read from the drive) is encapsulated by the controller and passed back across the network to the requesting computer.
As there is a possibility that these packets will be carried on an insecure network such as the public Internet, the IETF has required that iSCSI include IPsec encryption.
Part of the rationale behind iSCSI is that standardising on a single networking standard— TCP/IP — across the entire infrastructure yields economies of scale from a hardware and management perspective. It also increases the talent pool that can fill a position, as TCP/IP experience is more widespread than other technologies.
But Gelbart warns this could lead to organisational issues for IT management. What structures will be appropriate if the SAN connection mechanism becomes the responsibility of the network group rather than the storage group? There is much more to storage administration than the SAN fabric, but you don't want two centres of TCP/IP expertise. Is it feasible to merge the network and storage groups?
Wayne Glynne, storage business unit executive with IBM, says there should be room for some rationalisation between storage and network groups, but that would depend on the organisation concerned. But it is an issue that should be addressed, because "the most expensive part of a storage network is its management."
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The main attraction of iSCSI is that it provides an opportunity to connect servers to a SAN where this was previously ruled out due to the cost of Fibre Channel components. According to Bragg, a Fibre Channel SAN connection typically costs $8000 to $16,000, including the host bus adaptor, a fibre switch port, and storage management software. The reason the range is so broad is that many organisations have a policy that requires multiple paths between each server and the SAN. This sort of budget is regarded as reasonable for mission critical systems, but few organisations are prepared to pay that much to connect every server in the shop.
iSCSI is a much cheaper technology, costing from "a few hundred to a couple of thousand dollars," says Bragg. This makes it a cost-effective way of connecting secondary servers to a SAN. Extending a "double path" policy from Fibre Channel to iSCSI "is a contradiction to what iSCSI is all about—a low-cost infrastructure for secondary servers," he says.
Despite the savings, he says iSCSI is not appropriate for mission critical servers with latency and bandwidth requirements in order to meet agreed service levels. But the cost advantage means IT managers can reap the advantage of storage consolidation across a larger number of servers. iSCSI is "in our view, complementary to a fibre SAN," he says.
According to Bragg, iSCSI is not especially efficient even with gigabit Ethernet, so Fibre Channel will still be attractive for high-performance applications such as those involving Oracle databases.
Gelbart agreed that the initial takeup of iSCSI will be at the low end, as iSCSI over gigabit Ethernet is inferior to Fibre Channel (especially since 2Gbps Fibre Channel is already available). This may change when 10Gbps Ethernet becomes broadly available, he suggests, but 10Gbps Fibre Channel will probably appear around the same time and will again be the more efficient of the two.
Fibre Channel protocols can also be used over IP networks. Fibre Channel over IP (FCIP) allows Fibre Channel commands and data to tunnel through IP networks to connect geographically separated SANs. One disadvantage is that this still requires the use of relatively expensive Fibre Channel hardware.
Another situation where iSCSI's low cost can come into its own is for inter-site disaster recovery links, Bragg suggests. Using Fibre Channel for this purpose can be very costly, and he says that some organisations are looking at using iSCSI solely for this purpose. Gelbart disagrees, saying that while iSCSI is suitable for SANs, FCIP is more appropriate for replication.
In Australia, distance is the greatest challenge to the "storage on demand" model, says Glynne. Fibre Channel cannot reach beyond a metropolitan area, while iSCSI is practically distance-independent and telcos can provide the long-haul IP links.
Glynne observes that FCIP is a less mature technology than iSCSI, and that the performance of the two should be similar. Consequently, organisations should consider the skills they have before making a choice, he says. IBM has a foot in both camps: "the market will decide," says Glynne.
The low cost of iSCSI means pilot projects are affordable, so organisations can consider putting a toe in the water the next time a secondary server requires additional disk space. "We see iSCSI taking a big chunk out of the NAS market," says Bragg.
Although various vendors claimed to have sold iSCSI products in Australia, none could put forward a customer as a reference site for T&B to speak to. This suggests that the projects are either still at the pilot stage, or that the organisations concerned are secretive.
