SAN Building Blocks

W. Curtis Preston

Before I get into this month's topic, I'll review what I've covered so far. In the first article of this series, I discussed the reasons that SANs exist. Because this column is dedicated mainly to backup and recovery, I covered the ways that SANs make backup and recovery easier. The second and third articles in this series explained the basics of Fibre Channel, starting with Fibre Channel's advantages over parallel SCSI.

Although I did not use the term parallel SCSI in previous articles, I'd like to introduce it now. Since SCSI refers to both the physical medium and the protocol, we need a term that refers to "traditional" SCSI. In traditional (i.e., bus-attached) SCSI, SCSI data travels over several conductors in parallel. (SCSI cables range from 50 to 80 conductors in a single cable.) Therefore, we are referring to traditional SCSI as parallel SCSI.

In contrast, you will remember that Fibre Channel has only two conductors, one for transmitting and one for receiving. If SCSI traffic was meant to travel across several conductors in parallel, how does Fibre Channel accept SCSI data? The answer is the new SCSI-3 specification for the SCSI architecture, which is very different from its predecessors. One of the main differences is that the SCSI-1 and SCSI-2 specifications were laid out in a single document each, where the SCSI-3 specification consists of more than 30 documents describing a multi-layered architecture.

This allowed the specification of other layers, as long as each new layer followed the communication specifications of the existing layers above and below it. The Fibre Channel specifications (FC and FC-2) were then added to the SCSI-3 specification. (Read more about this at http://www.t10.org. T10 is the ANSI committee that develops the SCSI standard.)