Home >  Articles > Can IP storage deliver SANs for everyone?

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After enjoying considerable success with a one-terabyte storage area network (SAN) based on IBM BladeCenter blade servers and a Hitachi Data Systems Thunder 9200 storage system, Melbourne's Holmesglen Institute of TAFE had almost resigned itself to the costly task of adding a second SAN, on another campus, before it found an alternative.

Talks with HDS pointed out the potential value of iSCSI, an emerging technology for transporting SCSI commands across standard IP networks, as a replacement for SAN-standard Fibre Channel technology, whose reach is limited by the length and expense of fibre-optic cabling. Using iSCSI, the three-campus vocational school could extend its central SAN across all three campuses, and to four remote sites, over inexpensive gigabit ethernet connections that run into the institute's 34 Mbps wide area network.

It was an immediately attractive option simply because of the promised cost savings, says Ron Gascoigne, Holmesglen's IT manager. "We were going to have to continue in the way we had in the past, which was using Fibre Channel," he explains. "If we'd continued down the Fibre Channel path, we would have had to put FC switches into the BladeCenter at a cost of around $30,000. Whereas with iSCSI, I could put four gigabit ethernet links into the BladeCenter for $5000."

The considerable savings can instead be channelled into buying a second BladeCenter chassis, which will provide Holmesglen with functional redundancy and disaster tolerance. iSCSI is currently being used to link Windows servers into the SAN, but the institute will soon trial the technology for linking Novell and Linux servers too. Ubiquitous use of IP, which has eliminated the need for additional expensive Fibre Channel ports with each new server that's added, will also speed the introduction of IP-based services such as videoconferencing and voice over IP.

SANs for everybody

Originally designed to raise the bar for the consolidation and secure storage of data, SANs have been immensely popular: in the fourth quarter of 2003, IDC pegged shipments of external disk systems at 8.4% year-over-year to be worth US$3.7 billion for the quarter. The actual amount of storage shipped increased 52% over the previous year. SAN-attached network storage grew 26.4% in the quarter compared with the previous year, while NAS and SAN storage combined grew 23.5% to be worth US$2.1 billion, or 57% of the total external storage market.

While the concept of network-attached storage now dominates mainstream approaches to storage, however, smaller and many medium companies have continued to be left out of the SAN revolution because of the stubbornly high cost of Fibre Channel connectivity and SAN equipment.

While SANs clearly represent the direction forward for the storage industry, this exclusion has limited the market opportunities available to SAN vendors and presented often unworkable total cost of ownership equations for many companies that could truly benefit from SANs' new architectures. Many such companies have had to settle with IP-based NAS storage, which has its own merits but lacks the logical isolation of a SAN solution.

"The SAN is a very distinct propagation of Fibre Channel," says Simon Green, managing director of Network Appliance ANZ. "The original theory was that it was going to bring high-performance, consolidated storage together and lower the cost of ownership. However, it went completely the other way, became very difficult to manage, and customers ended up building very distinct [SAN] islands in their networks. Now they're trying to figure out how to extend their investments."

Recently, new product launches have suggested the future of storage is much more open than it has been, with NAS and SAN each taking on some aspects of the other and unified devices blurring the once significant boundaries between the two. Dual-headed storage arrays, for example, allow the combination of file-based and block-based storage in the same array, allowing the logical strengths of NAS and SAN to be delivered from a single storage array.

iSCSI, too, will be a major enabler in the push to bring SAN and NAS worlds closer together, resolving a major issue that has chronically inflated the cost of Fibre Channel SANs. With dozens or hundreds of servers potentially needing to be connected into the SAN, and ports often costing more than the servers themselves, this cost has meant only high-priority servers have typically been added to the SAN. Smaller, workgroup systems instead have been kept at arm's length, limiting performance and maintaining artificial barriers to data access.

Now that the IETF's recent acceptance of iSCSI has spurred support for it in mainstream operating systems and host bus adapters (HBAs), it's clear that the standard will become instrumental in opening up the world of the SAN - so long kept inside high walls as much by design as by technical need - to the broader enterprise. This openness, in turn, is redefining the SAN cost equation in a way that will open up the benefits of consolidated storage to companies of all sizes.

"IP amplifies the benefits of Fibre Channel SANs by breaking down the inherent architectural limitations of the SAN," says Tom Clark, director of SAN technology with SAN vendor McData. "Customers that have their very mission-critical, high-performance servers can now amortise the investment they've made in the high-performance SAN over much broader populations of Windows servers. They'll get the full benefit of shared storage, streamlined tape backup operations, and reduced administrative overhead."

For all its benefits, iSCSI does have one major drawback: translating between Fibre Channel and IP requires processing power in such volumes that it can introduce significant speed hits. By some accounts, iSCSI can consume fully one-third of the server's CPU power. This makes it problematic in high-volume, transaction-based environments but will have less impact on lower-end companies that aren't likely to be utilising the full bandwidth of a gigabit-class connection anyway.

Many companies also now offer iSCSI accelerators that use purpose-built application specific integrated circuits (ASICs) to speed iSCSI translation; expect this acceleration to become a standard part of SAN environments built on equipment that's increasingly shipping with dual ethernet and Fibre Channel ports.

Relearning disaster recovery in a SAN world

While iSCSI tackles SANs' traditional high costs, a second set of standards is working to dispel another traditionally frustrating aspect of SANs: their performance over a wide area.

Inherently linking SANs with the Fibre Channel they run over has presented a complex puzzle for companies wanting to provide disaster recovery between sites: although carrier fibre services can happily carry Fibre Channel signals across long distances with virtually no latency, those services have been prohibitively expensive for most organisations. In turn, DR strategies have been limited to short distances where the risk of proximity is balanced against the high cost of isolation.

Promising to change all this are iFCP (Internet FCP) and FCIP (Fibre Channel over IP), two related but discrete ways of pushing data across long distances using commodity IP networks instead of costly fibre.

FCIP is implemented by introducing an additional switch that links existing Fibre Channel SAN switches with an IP network; those switches act as bridges, translating Fibre Channel into IP for long-distance transmission and IP back to Fibre Channel at the receiving side. Data is tunnelled between sites, producing a single logical SAN that seamlessly spans the two sites and allows for the continuous data replication necessary for effective DR. FCIP can also carry FICON and ESCON mainframe traffic.

By contrast, iFCP replaces the Fibre Channel switches with iFCP-capable switches that connect Fibre Channel devices directly into an IP network. Whereas FCIP produces a logically homogeneous SAN spanning the two sites, iFCP terminates Fibre Channel sessions and converts the traffic into IP - creating logically discrete data sessions that can provide better error checking and logically isolate one SAN from the other. Because iFCP is implemented in traditional IP switches, it is a far lower cost solution than FCIP, which still requires separate Fibre Channel switches.

Whether customers use FCIP or iFCP, the end result is the creation of a storage environment that allows once tightly-controlled SANs to be seamlessly extended across great distances. This will overcome a significant issue for companies wanting a more resilient DR infrastructure, with lower-cost solutions providing more value for money and allowing on-site SANs to be replicated to third-party storage and disaster recovery sites with little overhead.

Although the convergence of IP and SAN provides architectural flexibility in SAN environments, Dr Kevin McIsaac, research director with META Group, cautions that it's not a full solution to resolving storage complexity. Often complex issues such as staff training and management systems also need to be addressed, he says, before customers jump into IP storage with both feet.

"The actual cost of storage hardware has declined 35% to 40% per annum," he says. "The hardware cost is not going to be the problem; the problem will be people. The idea of storage over IP was that a company could use IP networking staff to run [storage]. This is true, except that they don't know anything about storage policy, have no idea what a LUN (logical unit number) is, and don't actually understand anything about the storage networking side. iSCSI and FCIP are interesting technologies, but companies need to be extremely cautious about implementing them because they're going to introduce complexity."

The combination of iSCSI, FCIP and iFCP will ultimately change the definition of a SAN, allowing customers to focus more on its improved management capabilities and think less about the technological esoterica involved in getting SANs up and running. Vendors have demonstrated SANs that span oceans and continents, a practice that is likely to become more common as previously unheard-of economies of scale intensify the consolidation of storage into fewer and fewer global storage sites. Thanks to the separation of form from function, these changes will be able to be implemented easily and inexpensively.

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