Linking FC SANs over IP backbones

Posted on August 01, 2000

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Existing Internet Protocol (IP)-based WANs can be used to link Fibre Channel-based storage area network "islands."

By Brian Larsen

Most companies now consider storage to be a strategic resource that supports key business goals. Higher disk capacities and declining prices are enabling projects that require tremendous volumes of data. International Data Corp (IDC), a market research firm in Framingham, MA, estimates that in disk storage alone, access grew from 10,000 terabytes in 1994 to 116,000 terabytes in 1998. This is estimated to reach approximately 1,400,000 terabytes in 2002.

The ever-increasing need for storage presents a parallel challenge to implement systems that allow users to share, access, and protect information across the enterprise. Here again, the projections are staggering. IDC estimates that global spending on storage area network (SAN)-related solutions will grow from $250 million in 1998 to $2.4 billion in 2002. According to Strategic Research Corp., in Santa Barbara, CA, "SAN technology is moving into the mainstream of distributed networking, and will be adopted as the standard way of attaching and accessing storage within a few years."

The reasons for these projections are easy to pinpoint. A SAN offers the benefits of:

  • centralized management of dispersed, enterprise-wide storage
  • anytime, anywhere access to vital business data
  • high performance and high availability
  • server-less backup
  • scalability

Enterprise-wide storage also implies connecting over a wide area network (WAN). Historically, this has been accomplished using relatively expensive dedicated circuits. However, new developments are enabling transparent SAN connectivity over standard IP networks. A SAN-over-IP solution should:

  • Take advantage of an organization's existing or planned IP backbone to move storage data between SAN "islands," without intersecting with LAN traffic
  • Provide connectivity among servers and storage devices that use a variety of protocols, including the ability to merge protocols such as Fibre Channel, IP, and ATM for long-distance storage deployment

A couple of key technical issues must be solved before storage networking and SAN data services are broadly deployed over private or public IP networks. Progress is being made; here's an update on some major accomplishments.

  • Service quality. IP networks have improved dramatically in terms of robustness, performance, capacity, reliability, and stability. However, they still don't measure up to the consistency of cell-based-and more costly-dedicated network implementations. This is especially true of the public Internet. But, market pressure to develop steady and consistent connections with IP is building.
  • Dropped packets. Congested IP routers typically discard packets that can't be forwarded. Tolerable in some applications, this limitation can seriously affect performance and data integrity in a storage network environment. Tools are being developed that improve the ability to diagnose and troubleshoot the IP network and improve system performance to push more data through routers faster-without lost packets.
  • Security. Issues of access control, privacy, and authentication are paramount for companies seeking to move critical data over networks; IP was designed for openness and connectivity, not security. Network vendors are making strides in adding security features such as firewall and virtual pubic network (VPN) functionality to dedicated systems, general-purpose Unix servers, and as optional software in routers.
  • Network management. Running everything on IP makes management easier, but also requires the right set of management and troubleshooting skills and staff resources. For example, specialized tools need to be developed to handle the demands of data replication applications.

One approach to delivering SAN applications over IP is to use a multi-gigabit switching platform that extends storage networks-including Fibre Channel SANs-over WANs by integrating channel technology, high-speed network technology and open systems hardware and software in a single platform. This platform provides:

  • Operation over a routed IP network (private or VPN)
  • IP encapsulation/de-encapsulization, converting packetized data for travel over the IP network
  • A transport layer for packet-reordering and retransmission of dropped packets
  • Load balancing and alternate pathing over multiple IP interfaces or transport circuits-the same capability available on dedicated links, without the high cost

To understand this approach, it's helpful to understand that IP operates at Layer 3 of the Open Systems Interconnection (OSI) stack. This layer analyzes the addresses in packet headers and routes packets across subnetworks. This approach creates the need for an enhanced Layer 4 transport, which is functionally tailored for storage applications and uses Layer 3 for support. This transport layer communicates up to the application and down to the IP layer. It handles end-to-end connections and is responsible for packet ordering, safe delivery, flow control, and error correction, which is done transparently to the application.


IT departments can use existing storage components and infrastructures to create new backup applications.
Click here to enlarge image

Support for standard IP at the network layer facilitates interoperability with standard routers from vendors such as Cisco, Nortel, and 3Com. The router recognizes SAN/IP traffic as a standard IP packet. The IP header contains standard IP addressing information, and fully supports a router's data packet forwarding, including fragmentation and broadcast. The transport layer replaces TCP and establishes circuits within the IP network to make the data transmissions faster, more efficient, and reliable.

An important consideration in deploying SAN-over-IP applications is the inherent latency of the IP network. Network latency is the delay introduced when a data packet is momentarily stored, analyzed, and then forwarded. IP is well suited for asynchronous applications today, with a move towards "real-time" access when Quality of Services (QoS) is improved. Companies considering deploying SAN-over-IP should look to the applications that can tolerate some delay, such as remote tape backup and asynchronous disk mirroring.

SAN over IP in action

Applications that can tolerate the variable latency inherent in IP networking might include:

  • backup/restore
  • data replication
  • data warehousing
  • mirrored Web sites
  • data center consolidation
  • data migration
  • distributed data movement

With SAN-over-IP, data can be transmitted to geographically separated locations, reducing the risk of natural and site-wide disasters, thereby protecting mission-critical data and ensuring business continuance. For instance, Web servers located in different states or countries (for user-performance purposes) can provide congruous, current information to visitors by transferring and sharing updated information over an IP backbone. Or an IT department can use its existing storage components and infrastructure to create a new backup application [see diagram]. Another possibility is to use the existing IP infrastructure to migrate data between storage systems for use in testing projects. From the IT standpoint, an IP solution reduces bandwidth costs, speeds deployment, and provides easier network management.

In summary, SAN over IP can potentially offer these benefits:

  • Flexible, scalable, economic storage infrastructures that span the enterprise by providing connectivity across platforms and protocols (Fibre Channel, IP, ATM).
  • Reduced costs because businesses can leverage their existing IP infrastructure to access bandwidth to store data across their enterprise without dedicated lines.
  • Increased time-to-market capabilities due to IP flexibility, allowing IT departments to respond more quickly to requests for new capabilities and applications.

Implementing SAN applications over an IP backbone leverages the existing IP-based network infrastructure, delivering quicker implementations, reduced bandwidth costs, and easier and less costly network support and management. Advances in telecommunications and network management hardware and software will make SAN-over-IP solutions available to an increasing number of organizations over private networks, VPNs, and the public Internet.

Brian Larsen is director of product management for open systems at Computer Network Technology (CNT), in Minneapolis, MN. www.cnt.com.


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