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Fourth generation storage networking: one decade of Fibre Channel.

Storage networking started even before the Fibre Channel standard was adopted in 1994 by ANSI. In the 1980s, IBM introduced an architecture to share storage among multiple mainframes through ESCON directors and interconnections.

The need to share data on a casual scale is well done with Ethernet and LANs, but it wasn't enough: hence the development of storage networking through Fibre Channel storage area networks (SANs). Now the industry is on the verge of another milestone, the move to 4 Gb/s speeds in time to celebrate the start of the second 10-years of Fibre Channel.

Generation One: 1 Gb Loop

Fibre Channel is a carefully considered specification with both protocol and addressing flexibility, with Fibre Channel Arbitrated Loop (FC-AL) being the first 8-bit increment in Fibre Channel addressing.

The first 1 Gb/s Fibre Channel SANs were typically simple loop configurations to share data. However, loop addressing has several technical shortcomings such as scalability and manageability that tended to cancel out their cost advantage over a Fibre Channel fabric. Loop topology (as opposed to addressing) is performance challenged since the available bandwidth is shared among the nodes on a loop. Customers were experienced in networking from the Ethernet world, which created expectations of being able to create more expansive storage networks than those based only on loop-address-based hubs and loop switches.

Generation Two: 1 Gb Fabric

As more Fibre Channel fabric options became available adoption spread to larger customers who got better management tools and, as standards matured, improved interoperability and better support. The customers who liked SANs wanted to move aggressively into bigger SANs with requirements for cascading to create larger networks of storage and servers. This approach is very difficult in loop configurations, but appropriate for switched fabric configurations.

Generation Three: 2 Gb Fabric

The arrival of 2 Gb/s Fibre Channel provided more ways to connect and introduced many more networking tools into the SAN toolbox, such as trunking and using backbone architectures to allow each speed to be used for best effect. With backwards compatibility, the speeds of the network could be mixed without forcing a major problem for the IT people implementing the SAN. With the advent of the higher speed, standards also improved to provide better interoperability and standardized ways of trading information to make SANs easier to manage.

Generation Four: 4 Gb

With the advent of 4 Gb/s Fibre Channel connections the question is how to use it to best effect. New networking speeds are commonly used first as an aggregator link to relieve congestion on busy links already in existence. As the costs drop on the new speeds, the higher speeds connect end nodes in the network and existing equipment usually migrated to less demanding applications. What happens in the real world is that new equipment goes to the faster speed because few can afford to keep touching their storage network. The backwards compatibility and mature interoperability make this easy to implement.

Storage Networking Options

There are several aspects of storage networks and as the industry matures there will be several ways to connect storage:

* Topology: large and multitiered (fabric) or simple and single tiered (loop)

* Format: block storage (SAN) or file storage (NAS)

* Transport: Fibre Channel or IP (iSCSI)

* Protocol: SCSI- the application doesn't see any difference, which is key, as almost all applications want to talk to external storage using SCSI commands, so the fact that Fibre Channel and iSCSI allow you to transport SCSI commands makes SANs usable to existing applications. The end-user has no idea about the network.

Most IT shops will opt for a mixture of these options, since each has its strength and optimum use. Start with the application and let it dictate the technology. Databases need block storage as do any applications that need to control their storage resource commonly found in SANs. File storage as found on Network Attached Storage (NAS) can be quick to install and configure for applications such as e-mail and others where file structure can be imposed by the storage device.

For single node connections, traditional loop is adequate. Connections to less demanding end node devices could comprise a 1 Gb Fibre Channel or iSCSI connection. To quickly add capacity for non-file-specific applications, a NAS device has proven to be a popular alternative over 1 Gb/s Ethernet. As the demand for increasingly more complex management and/or speeds appears then high speed Fibre Channel is a good choice for a storage network.

4 GB Storage Networking

4 Gb/s Fibre Channel SAN implementers initially expect to see host bus adapters and cabling for their servers, followed by SAN switching connecting to Fibre Channel storage arrays. There may be a small premium initially for the 4 Gb/s parts, so the savings of purchasing 2 Gb/s parts today and replacing them with 4 Gb/s parts when the total solution is ready has to be considered against the labor of doing things twice. 4 Gb/s will be backwards compatible with earlier 2 Gb/s and 1 Gb/s Fibre Channel products. Most IT shops will just start plugging in 4Gb equipment when it is ready. Expect to see 4 Gb/s solutions arrive in the market in 2005.

Storage Networking Futures

The introduction of 4 Gb/s Fibre Channel makes a difference on other technologies as well. For instance when making a decision between Fibre Channel and iSCSI the difference in performance becomes profound if one is considering the choice between 4 Gb/s Fibre Channel and 1 Gb/s iSCSI. Similarly, with 1 Gb/s IP connections to NAS devices, there is a considerable performance penalty to pay for something other than Fibre Channel.

The decision between 10 Gb/s iSCSI, 10 Gb/s NAS over IP and 8- or 10-Gb/s Fibre Channel is more complicated. For backwards compatibility reasons, 8 Gb/s Fibre Channel will probably win out over 10 Gb/s Fibre Channel. In comparing Fibre Channel against the Ethernet based solutions of NAS and iSCSI there are other considerations, such as economics, support staff and the maturity of the management tools. Fibre Channel storage management is the most mature for storage applications and the next generation of Fibre Channel at 8 Gb/s is expected to be backwards compatible with 4, 2 and 1 Gb/s Fibre Channel. This is unlike 10 Gb/s IP, which is not compatible with previous generations of 1 Gb/s IP, making migration very deliberate and somewhat costly.

If you have a large SAN requiring the best possible management tools, the Fibre Channel SANs are still the best answer for the overall structure, but they will co-exist with other technologies appropriate to their applications. For simpler connectivity in a large environment, 1 Gb/s iSCSI will offer connectivity familiar to the IT staff and for some applications, the performance may be "good enough". The availability of storage arrays and tape libraries with high-speed IP connections does not approach that of Fibre Channel, so that may help you decide on timing for deployment of your iSCSI SAN. And based on what is available in 2005, you would have to choose between 4 Gb/s Fibre Channel or 1 Gb/s Ethernet based networks. Finally, for file-based systems, 1 Gb/s IP links for NAS provides an increasingly attractive way to inexpensively and quickly add capacity for some applications that are not file sensitive. Management of NAS devices continues to be more of a challenge long term and making sure data is regularly backed up is critical.

Another consideration in SAN design is that of distance. IP is relatively insensitive to the distances required to take a packet of data. Fibre Channel, on the other hand, has a limited distance that requires a cooperative link for remote replication, migration or other disaster recovery applications. This cooperative link, for example Ethernet or SONET, working with the SAN can take some subset of the data to reduce the bandwidth required to another site to protect or share data

Storage Networking is bringing the world a more efficient way to manage storage. IT shops that want to provide more productivity from the people they already have will be required to manage their storage as a pool, since it simply isn't practical to try to manage thousands of separate servers with direct attached storage. Advances in standards to ease management, faster storage networking speeds and improved storage management tools make storage increasingly networkable. Considerations of scalability, performance and management are the primary determiners of which technologies to utilize, with most storage networks comprising more than one technology. For example, many IT shops will choose a 4 Gb/s Fibre Channel backbone architecture with sites linked by SONET or 1 Gb/s IP connections and remote departments eventually connected with iSCSI. NAS will continue to find a home for those seeking the easiest way to add storage in a hurry. Bottom line: don't let the variety of options deter you, to get the most out of your storage investment in people and equipment you will need to network your storage, so get going.

Erik Ottem is director of marketing at Agilent Technologies Inc., Palo Alto, CA
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Title Annotation:Connectivity
Author:Ottem, Erik
Publication:Computer Technology Review
Date:Jun 1, 2005
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