Seagate 15K.2 Inflection Point - The New Era of Serial Attached SCSI - Page 1

Technology Paper SCSI Inflection Point: Standardizing on Serial Attached SCSI (SAS) Introduction Necessity can indeed be the mother of invention, and over 20 years ago it gave birth to an innovative interface that went on to achieve worldwide success: parallel SCSI. To be sure, the authors of the original parallel SCSI standards well understood the theoretical superiority of serial interfaces, with their inherently simpler and more robust architecture. Unfortunately, serial technologies at the time were woefully slow, adequate for the pedestrian needs of peripheral devices (for example, keyboards and mice) but far too sluggish to efficiently move the multimegabyte files increasingly found on primary storage devices. And so parallel SCSI (and its desktop counterpart, parallel ATA) came into being; not because parallel interfaces are fundamentally preferable (quite the contrary), but simply because they were the best option at the time, given the limitations of available technology. The premise, and promise, of parallel architecture was certainly compelling: to bypass the performance bottleneck of a single signal path (sending bytes serially, or one bit after another), just add more signal conductors to enable a byte's multiple bits to be sent concurrently (or in parallel), each on its own separate data path. The simple logic seemed unassailable-an eight-lane highway can certainly move far more traffic than a single-lane one. To be sure, practical implementation of parallel interface theory proved decidedly complex, with numerous technological hurdles to overcome. Nevertheless, the fact remains that for its time and purpose, parallel SCSI was a profoundly important advancement in storage interface technology. But, of course, times change.