Simplified storage management, narrow backup windows, and better application performance are some of the benefits
As e-business initiatives and growing demand for digital information force IT departments to expand storage resources while maintaining 24 x 7 access, managing storage assets efficiently becomes extremely important. Yet, managing storage that's directly attached to servers becomes increasingly difficult, costly, and time-consuming as the number of servers grows. Thus, IT departments are looking to Storage Area Networks (SANs) and Network Attached Storage (NAS). Both SANs and NAS simplify the storage-management task by consolidating server storage into central storage arrays that you can manage from one administrative console.
In contrast to NAS systems, which connect to your LAN and provide file-level access for client systems, SANs are dedicated 1Gbps or 2Gbps storage networks that connect disk arrays, servers, tape libraries, and other peripherals to a switching fabric or hub. SANs provide block-level access to data residing on shared storage arrays. Moving storage traffic onto a dedicated high-speed network can narrow backup windows and improve application performance by reducing contention for LAN bandwidth. And SAN management software can easily perform tasks such as reallocating storage space among servers without interrupting access to data. In addition, many NAS vendors offer products that can share SAN-based storage (for more information about such NAS products, see "Storage Networking," February 2002, InstantDoc ID 23563).
The Proven Standard
SANs are built around Fibre Channel technology because of its high speed (as fast as 2Gbps) and reliability compared with Ethernet networks. Companies such as Compaq, EMC, Hewlett-Packard (HP), IBM, and Nishan Systems provide full SAN solutions, frequently including networking components from third-party vendors such as Brocade Communications Systems, McDATA, Gadzoox Networks, QLogic, Adaptec, and Emulex. When you buy a SAN from a major solution provider, you can be confident that software and hardware will work as expected. However, assembling a SAN from various suppliers' components will require more effort because interoperability of third-party Fibre Channel SAN components is a work in progress.
Fibre Channel SANs are expensive and complex to implement and manage, so they've been implemented primarily by large enterprises that have inhouse technical expertise and challenging storage-management environments. A relatively large SAN that supports hundreds of users and has several Fibre Channel directors (switches with many ports and redundant backplanes) and storage arrays can cost $1 million or more. Even a small SAN that has an arbitrated loop hub and one storage array and supports no more than 20 servers can cost a few hundred thousand dollars.
Interoperability has also been a problem. Until recently, assembling a multivendor Fibre Channel SAN was extremely difficult because vendors have interpreted Fibre Channel standards differently and have used proprietary features to their competitive advantage. For example, you couldn't assume that different vendors' Fibre Channel switches would interoperate or that a particular vendor's SAN management software would work with another vendor's switches, host bus adapters (HBAs), or storage arrays. But as I explain later, that situation is beginning to change.
An IP-Based Approach
Concerns about interoperability, the cost of Fibre Channel SAN implementations, and the need for highly trained IT staffs and specialized Fibre Channel network-management tools have spurred interest in the potential of IP-based storage networks, particularly as the price of Gigabit Ethernet switches has dropped. As a result, storage vendors are lining up to support the pending complementary protocol standards from the Internet Engineering Task Force (IETF): Internet SCSI (iSCSI), Fibre Channel over IP (FCIP), and Internet Fibre Channel Protocol (iFCP).
The iSCSI protocol lets IP networks carry SCSI commands and storage data, making IP-based SANs feasible. FCIP lets IP networks carry Fibre Channel control codes and data, so you can use IP links to connect geographically separate Fibre Channel SANs, as Figure 1 shows. The iFCP protocol lets Fibre Channel SAN components use lightweight gateways to connect to IP networks.
Depending on the level of performance you need, your budget, and how you use your existing network, you can add a storage array to an existing Gigabit Ethernet backbone (after installing iSCSI HBAs in each server) to implement an iSCSI SAN, or you can create a totally separate back-end network, as most enterprises do in their Fibre Channel installations. Companies that have a Fibre Channel SAN can also implement an iSCSI SAN and link the two SANs with a storage router that supports both protocols, as Figure 2 shows.
The three new protocols should be ratified later this year, and IBM, Nishan Systems, Cisco Systems, and others have already introduced storage products based on them. Whether first-generation iSCSI products will interoperate smoothly with one another remains to be seen, but vendors insist that the maturity of TCP/IP technology and the testing and refinement work being conducted by the Storage Networking Industry Association (SNIA) IP Storage Forum's member companies will ensure a high level of interoperability.
The Solution Is in the Cards
IP SAN technology is an attractive alternative to Fibre Channel. However, IP technology presents TCP/IP and iSCSI protocol overhead problems that vendors still need to overcome. Server CPUs currently handle the overhead on Gigabit Ethernet LAN segments, but a SAN's high-volume, block-data traffic increases this processing overhead and can substantially slow server performance.
Several vendors have introduced iSCSI HBAs with TCP/IP off-load engines to address this problem. Some products, such as Adaptec's ASA-7211 iSCSI Adapter, off-load all processing overhead; others, such as the Alacritech 1000x1 Single-Port Server and Storage Accelerator, partially off-load processing and relegate control and error handling to the server's CPU. These design choices stem from vendors' differing opinions about what percentage of dropped packets—and the corresponding CPU overhead that retransmitting them causes—will be incurred at a SAN's high traffic volume. Which approach is better remains to be seen. Pricing for the Adaptec card wasn't available at press time, but the Alacritech product lists for $999—about the same price as a Fibre Channel HBA. Most such cards also combine Gigabit Ethernet NIC functionality. Depending on whether a vendor's lineage traces to storage or networking, these cards are called HBAs or NICs.
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