Section 4.1. The Need for Fibre Channel

   

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4.1 The Need for Fibre Channel

In general, Fibre Channel is an attempt to combine the best of both worlds : channels and networks. Channel is a term that was first used in the mainframe world and denotes a structured, defined mechanism for accomplishing data transfer. In most cases the data transfer is between a computer system and a peripheral, such as a disk or tape unit. Well-known examples of channels include SCSI (Small Computer System Interface) and HIPPI (High-Performance Parallel Interface). Channels are typically implemented mostly in hardware.

Compared to a channel, a network is a much more general-purpose mechanism for accomplishing data transfer, and it is much less structured. A network can also typically operate over a much longer range and interconnect many more devices than a channel. Unlike channels, networks are implemented much more via functionality in the software layer than in the hardware layer.

One approach to bringing together the storage and networking worlds is to take a network-centric view and extend the technology to add value and overcome the deficiencies. In general, this is the approach taken with the IP storage technologies described in Chapter 8.

The other approach is to take a storage-centric (channel-centric) view and try to extend the technology. This is the approach taken by Fibre Channel. One big advantage of Fibre Channel over IP storage is that Fibre Channel technology “based products have been shipping for almost a decade , whereas IP storage “based commercial products are a relative newcomer.

Given that Fibre Channel is an attempt to take a channel-centric view, reviewing the deficiencies of one of the most famous channel protocols, SCSI, will be useful. SCSI's deficiencies are as follows :

• It has a maximum data transfer rate of 80MB per second (subsequently raised to 320MB per second, but only after the advent of Fibre Channel), which is not enough for high-volume data storage.

• It allows a maximum of 16 devices per adapter, which is simply not enough.

• One of its greatest strengths is also one of its limitations. SCSI has evolved over the years to meet growing demands, yet SCSI vendors have also managed to build in backward compatibility. The problem is that this places a requirement on administrators to ensure that no older SCSI devices are around, because if they are, the bus reverts to the least common denominator.

• It supports a cable length that can be measured at the most in terms of tens of meters . This is simply not enough for clustering, geographical separation of clusters, and the like.

• Other alternatives to SCSI, such as Serial Storage Architecture (SSA), do exist, but these have not really taken off in the Intel architecture servers or as an open standard at all.

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