Getting Started: Understanding The Terminology of iSCSI vs. Fibre Channel
Storage Area Network
Storage Area Network (SAN) is a high-speed sub network of shared storage devices. A storage device is a machine that contains nothing but a disk or disks for storing data. A SAN’s architecture works in a way that makes all storage devices available to all servers on a LAN or WAN. As more storage devices are added to a SAN, they too will be accessible from any server in the larger network. In this case, the server merely acts as a pathway between the end user and the stored data. Because stored data does not reside directly on any of a network’s servers, server power is used for business applications, and network capacity is released to the end user.
Short for Internet SCSI, iSCSI is an IP-based standard for linking data storage devices over a network and transferring data by carrying SCSI commands over IP networks. iSCSI supports a Gigabit Ethernet interface at the physical layer, which allows systems supporting iSCSI interfaces to connect directly to standard Gigabit Ethernet switches and/or IP routers. When an operating system receives a request it generates the SCSI command and then sends an IP packet over an Ethernet connection. At the receiving end, the SCSI commands are separated from the request, and the SCSI commands and data are sent to the SCSI controller and then to the SCSI storage device. iSCSI will also return a response to the request using the same protocol.
A serial data transfer architecture developed by a consortium of computer and mass storage device manufacturers and now being standardized by ANSI. The most prominent Fibre Channel standard is Fibre Channel Arbitrated Loop (FC-AL). FC-AL was designed for new mass storage devices and other peripheral devices that require very high bandwidth. Using optical fiber to connect devices, FC-AL supports full-duplex data transfer rates.
The Promise of iSCSI
The oft-repeated promise of iSCSI is relatively uniform across many storage vendors and analysts. An iSCSI IP storage network is generally viewed as less complex to implement and maintain than Fibre Channel. There is also a significant cost differential between iSCSI and Fibre channel, though iSCSI currently does not scale to the same performance level as a Fibre Channel SAN.
What does iSCSI Mean for Your Storage Network?
As opposed to straightforward NAS, iSCSI also holds a significant advantage in that it shows storage as a virtual block-level device, which allows applications to operate under the illusion that the storage space being used is exclusive. This is important for many different types of data-intensive applications. The question of whether iSCSI was an early victim of hype gets a very different response depending on whom you ask. Some see it as evolving in line with expectations, while others believe that some of the early enthusiasm was misguided.
|Storage Milestones and Predictions|
|1994||First Fibre Channel specification|
|2000||Fibre Channel specification update for 2 Gbps|
|2003||iSCSI specification ratified as a standard|
|2004||IBM ‘re-introduces’ iSCSI to its line-up with its DS 300 product|
|2005||IDC projects that the iSCSI market segment could grow to approximately $2.48 billion|
|2007||iSCSI forecast by Gartner to become dominating SAN attach protocol|
iSCSI-based SAN vs. Fibre Channel
The differences between in an iSCSI-based SAN and Fibre Channel, in terms of performance and cost, is in the limitations of Fibre Channel itself. While Fibre Channel is a high-performance transmission technology optimized for the same block storage format that storage devices use, it does have drawbacks:
[Adapted from What does iSCSI Mean for Your Storage Network?]
Total Cost of Ownership (TCO): The Total Cost of Ownership (TCO) for operating a Fibre Channel SAN, while lower than the DAS model, is still high. Since organizations vary widely in their storage needs, it’s difficult to develop a set of assumptions for generating average cost figures. Still, Fibre Channel is a fairly new technology and many IT staffs have limited Fibre Channel expertise. Finding the necessary specialized personnel can be challenging, and training is often not readily available. As a result, installing and maintaining a Fibre Channel network is typically difficult and expensive.
Operating Distance: Although the theoretical limit for Fibre Channel is 10km, individual multi-mode fiber links used in Fibre Channel SANs may have a practical limitation of 250 to 500 meters. The storage ecosystem is evolving to where large organizations often have SANs located far from the LAN, to provide geographical redundancy as part of disaster planning. This means even 10km may be increasingly inadequate.
Who’s Using It?
From IBM’s point of view, the environments where iSCSI is being deployed are typically SMBs, although some departments in larger businesses are also adopting iSCSI. EMC agrees that the SMB market is showing interest in iSCSI, but also notes that larger enterprises are showing an interest as well.
“Many larger enterprises are evaluating iSCSI as a complementary solution to network-isolated servers and workgroup-level storage,” EMC’s Joyce said. “iSCSI also fits well within an enterprise information lifecycle management (ILM) strategy that calls for tiered storage.”
Cisco’s Ed Chapman said he sees iSCSI as a cost-effective interconnect technology that’s relevant for enterprises of all sizes. “Our SAN switching products are designed to be gateways between iSCSI servers and Fibre Channel storage devices such as disk arrays and tape libraries,” Chapman explained. “So, our target market for iSCSI are customers who have already made an investment in Fibre Channel SANs and are considering iSCSI as a way to attach lower-cost servers to those SANs.”
Chapman said that many customers he’s spoken with have a majority of servers that are not attached to a SAN, the primary reason being the prohibitive cost of equipping them for Fibre Channel.
The Cisco experience of using iSCSI as an interconnect technology is also something that it shares in part with Microsoft. According to Microsoft’s Lorenson, among its customers “usage varies from new, first SAN all the way to a full enterprise Exchange deployments on iSCSI as well as environments where iSCSI is used to connect to existing Fibre Channel resources for the low price band servers.”
Sean Michael Kerner is senior editor for InternetNews.com, the news service of Internet.com
This article was originally published on September 08, 2006