MarketEye: Disk Drive Interconnect Technologies: Serial Attached SCSI

06.11.2004 // Posted by: // Posted in: Articles

Statements of fact and or opinions expressed in MarketEye by its contributors are the responsibility of the authors alone and do not imply an opinion of the officers or the representatives of TTI, Inc.

Ronald Bishop	June 11, 2004



In this edition of MarketEye, Ronald Bishop reviews the ten newest technologies and explains which connectors make them possible.

Serial ATA made improvements in data integrity and is now able to compete with the Small Computer Systems Interface (SCSI) in the enterprise market. The SCSI community has responded and is pushing Serial Attached SCSI (SAS). Unlike SATA, SAS also defines interconnections between separate enclosures, which is typical in enterprise environments. SAS chose a thumbscrew version Fujitsu’s MicroGigCN, the connector that was originally developed for Infiniband.

Enterprise disk drives historically have used an attachment technology called SCSI (Small Computer System Interface) as opposed to the ATA (Advanced Technology Attachment) used by the PC. The reason is that the SCSI drives are typically considered to be more “robust”. They have error detection capabilities, which are not present in ATA drives, and additional capabilities to make them more reliable. The speed of the drive was also a factor.

Serial Attached SCSI When the PC industry changed from ATA drives to the newer technology Serial ATA (SATA) drives, some of that advantage changed. The high-speed serial interface on the ATA was supplemented with improvements in error detection and of course the speed of the interface changed. As a result, SATA drives were closer in performance and even offered some advantage over the SCSI drives. Low cost RAID (Random Arrays of Inexpensive Disks) systems began to spring up, often to replace slower tape drives, but sometimes in place of SCSI systems.

In response to this encroachment into SCSI territory, a working group was formed to develop a Serial Attached SCSI (SAS) disk drive. It would be faster and more robust and again claim its leadership position in the enterprise storage market. It even acknowledged the SATA performance improvements by maintaining a degree of commonality with the SATA drive interface. The SAS architecture will allow SATA drives to be connected and managed in a SAS system. This allows the use of SATA drives in applications that are not critical (such as back up). It also allows a company to start out with SATA drives and then move to SAS when business conditions and resources allow.

Because SATA drives were developed for use internal to PC enclosures, initially, an external cable was not defined for SATA. SCSI drives have often been located in external cabinets, away from the actual servers to which they are connected. As such there have always been an external cable standard associated with SCSI. The cable system used a connector called the VHDCI. For SAS the external cable connector chosen was a version of the connector originally developed for InfiniBand. The original design was a product by Fujitsu called the MicroGigaCN. A thumbscrew-actuated interface differentiates it from the lanyard style latching mechanism of InfiniBand.

SAS differs from SATA also in the fact that SATA is intended to operate in direct connection to the processor via a dedicated port on the PC motherboard. SAS has the ability, using a device called an edge expander to connect to 128 individual devices and, using a fan out expander to interconnect to other edge expanders to allow the addressing of 16,256 devices in a single SAS domain.

SAS will face competition from SANs, either Fibre Channel or iSCSI. SATA will challenge SCSI for entry-level server storage.

Bishop's Top 10 Newest Connector Technologies | TTI's Connector Gateway