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Special Report: Serial Attached SCSI

One of the most comfortable and recognizable technologies to both server and storage admins is SCSI, which for 20 years has offered us a fast and flexible hard-disk interface. When originally conceived, SCSI was the first interconnect touted not just for disk access but also for printers, scanners, external floppy drives and even keyboards. Its very name--Small Computer System Interface--spoke to its potential to replace keyboard and printer ports, much as USB does today. As it turns out, the only popular use for SCSI outside of storage devices is scanners, but as a storage interface, SCSI shines like the full moon on a cloudless night.

Enterprises and high-end hobbyists prefer SCSI for internal and external storage, but time has caught up to conventional parallel SCSI. Parallel signaling technology has reached a practical limit due to signal bleed-over. Its latest incarnation, Ultra320, uses cables so thick with shielding they're difficult to work with and expensive. Longer, external differential SCSI cables are even more costly. Now, high-speed serial silicon has breathed new life into SCSI just as it breathed new life into the desktop ATA (sometimes referred to as IDE) standard. This next incarnation, SAS (Serial-Attached SCSI), provides considerable speed and usability improvements while maintaining compatibility with parallel SCSI.

Thinner Is Better

SAS's advantages start in the physical world with its cable architecture. SAS takes its internal cabling from SATA, the serial update to conventional ATA drives now referred to as PATA (Parallel Advanced Technology Attachment). These cables are significantly narrower with far fewer pins than SCSI cabling, and are keyed to prevent being inserted into SATA drives or controllers, which makes it easier for vendors to manage server airflow and cooling issues. The external versions of these cables are much smaller and cheaper than older parallel SCSI cables, and incorporate the same plug specification as InfiniBand, which was chosen due to the available overhead in the specification and the accessibility of cabling.

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