What are SANs and NAS?

Throughout the history of computing, people have wanted to share computing resources. The Burroughs Corporation had this in mind in 1961 when they developed multiprogramming and virtual memory. Shugart Associates felt that people would be interested in a way to easily use and share disk devices. That's why they defined the Shugart Associates System Interface (SASI) in 1979. This, of course, was the predecessor to SCSI - the Small Computer System Interface. In the early 1980s, a team of engineers at Sun Microsystems felt that people needed a better way to share files, so they developed NFS. Sun released it to the public in 1984, and it became the Unix community's prevalent method of sharing filesystems. Also in 1984, Sytec developed NetBIOS for IBM; NetBIOS would become the foundation for the SMB protocol that would ultimately become CIFS, the predominant method of sharing files in a Windows environment.

Neither storage area networks (SANs) nor network attached storage (NAS) are new concepts. SANs are simply the next evolution of SCSI, and NAS is the next evolution of NFS and CIFS.

History

As mentioned earlier, SCSI has its origins in SASI, defined by Shugart Associates in 1979. In 1981, Shugart and NCR joined forces to better document SASI and to add features from another interface developed by NCR. In 1982, the ANSI task group X3T9.3 drafted a formal proposal for the Small Computer System Interface (SCSI), which was to be based on SASI. After work by many companies and many people, SCSI became a formal ANSI standard in 1986. Shortly thereafter, work began on SCSI-2, which incorporated the Common Command Set into SCSI, as well as other enhancements. It was approved in July 1990. Although SCSI-2 became the de facto interface between storage devices and small to midrange computing devices, not everyone felt that traditional SCSI was a good idea. This was due to the physical and electrical characteristics of copper-based parallel SCSI cables. (SCSI systems based on such cables are now referred to as parallel SCSI, because the SCSI signals are carried across dozens of pairs of conductors in parallel.) Although SCSI has come a long way since 1990, the following limitations still apply to parallel SCSI:

• Parallel SCSI is limited to 16 devices on a bus.
• It's possible, but not usually practical, to connect two computing devices to the same storage device with parallel SCSI.
• Due to cross talk between the individual conductors in a multiconductor parallel SCSI cable, as well as electrical interference from external sources, parallel SCSI has cable length limitations. Although this limitation has been somewhat overcome by SCSI-to-fiber-to-SCSI conversion boxes, these boxes aren't supported by many software and hardware vendors.
• It's also important to note that each device added to a SCSI chain shortens its total possible length.