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Serial interfaces need constant shift

Posted: 01 Feb 2007 ?? ?Print Version ?Bookmark and Share

Keywords:iSCSI? Fibre Channel? RAID?

Leif: While CPUs and processing technology appear to yield new products almost every month, storage interfaces lag behind.

Technology advancements seem to proceed at unprecedented rates. However, while CPUs and processing technology appear to yield new products almost every month, storage interfaces lag behind.

SCSI interfaces were the most common choice several years back. It was a standard interface among most enterprise-class systems and mainframes, and was the basis for RAID technology in its infancy. The unique feature that SCSI brought was its ability to have multiple drives on one controller. Sure, we can do that now with almost any interface, but it was SCSI that pioneered the concept. In its first generation, up to seven drives could be daisy-chained together; it then evolved to 15. Soon, controllers with built-in multiple interfaces were introduced, making it possible to have as many as 60 drives on one controller, appearing as one drive. A decade ago, drives were much smaller, which meant users could have what was considered at that time to be huge storage pools.

SCSI is marching on, and in its latest spin-off, Serial Attached SCSI (SAS), more flexibility and greater scalability are key values. Yet, its acceptance has been slow, and while SCSI has become more popular over the years, it quickly became apparent that data requirements and throughput needs were surpassing the abilities of SCSI. While SAS is addressing some of these issues, it remains unclear how accepted it will become as most SAS implementations still use Fibre Channel connectivity to the host.

In theory, Fibre Channel was poised to launch where SCSI left off. One of the most unique features of Fibre Channel is that it allows up to 126 devices on a single interface. This enables solutions with unthinkable amounts of storage and limitless performance. Couple that with host bus adapters and controllers featuring dual- and quad-port interfaces, and the level of redundancy and capacity at a user's fingertips is unthinkable.

Another feature of Fibre Channel is that it uses a dense optical connection. Unlike SCSI, which uses a much larger cable and has cable length limits that range from 7ft to 12m with Low Voltage Differential (LVD) SCSI, Fibre Channel uses optics to interface with the host, and supports cable lengths in excess of 3,000m, providing greater flexibility when designing a data center.

Fibre Channel can take advantage of switches, allowing not only multiple host systems for connecting to a central storage device, but also enabling multiple arrays as well as other devices to coexist on what we refer to today as a storage network, something that other interfaces are unable to achieve. Fibre Channel marries the best of both worlds. Its ability to maintain backup devices on its storage network secures large amounts of data within hours, rather than what would ordinarily take days over a gigabit interface.

Another unique approach is iSCSI. It uses SCSI protocols such as Fibre Channel, but instead of using a proprietary interface to attach to a computer, it connects via ordinary Ethernet connections. It is similar to Network Attached Storage (NAS), but instead of having a mapped network storage device, an iSCSI device appears as though it is a drive attached directly to the system. Its current pitfall is the limiting performance of today's gigabit networks. With 10GbE currently lobbying for acceptance, its costs are still prohibitive to the masses. Time will tell whether or not iSCSI combines practicality, ease of integration and flexibility. It only needs the performance boost that can be gained with 10GbE to catapult it into popularity.

- Scott Leif
Globalstor Data Corp.

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