HP DL360 Serial Attached SCSI storage technology, 2nd Edition - Page 2

Abstract This technology brief describes Serial Attached SCSI-2 (SAS-2) technology, the evolution of the SAS standard, interoperability with Serial Advanced Technology Attachment (SATA) devices, enhancements to SAS-2 cabling and connectors, and SAS topologies and zoning. Transition from parallel to serial SCSI protocol The original SCSI standard was developed in 1981 to provide a common interface that could be used across all peripheral platforms and system applications, such as Redundant Array of Independent Disks (RAID) storage. Since that time, there have been seven generations of the parallel SCSI protocol. Each generation doubled the bandwidth of the previous one, primarily by doubling the bus clock frequency. But as the bus frequency was increased with each new generation, so did the negative impact of bus contention, signal degradation, and signal skew-slight signal delays from one wire trace to the next. After the development of Ultra320 SCSI with a bandwidth of 320 MB/s per channel, further bandwidth improvements to parallel SCSI could not occur without developing new and expensive technologies. In 2001, HP (Compaq), IBM, LSI Logic, Maxtor, and Seagate founded the Serial Attached SCSI Working Group to define the rules for exchanging information between SCSI devices using a serial interconnect (SAS). SAS was later transferred to the InterNational Committee for Information Technology Standards (INCITS) T10 to become an American (ANSI) and international (ISO/IEC) standard. SAS inherits its command set from parallel SCSI, frame formats and full duplex communication from Fibre Channel, and it uses the SATA interface for compatibility and investment protection. The SAS architecture solves the parallel SCSI problems of bus contention, clock skew, and signal degradation at higher signaling rates, thereby providing performance headroom to meet enterprise storage needs for years to come. SAS terminology Table 1 provides a list of terms related to SAS technology to aid in understanding the concepts described in this paper. Table 1. SAS terminology Term Definition ANSI Domain Enterprise-class devices Entry level drives Expander Initiator American National Standards Institute An I/O system consisting of a set of SAS devices that communicate with one another by means of a service delivery subsystem SAS drives that provide maximum reliability, highest performance, scalability, and error management for use with unconstrained I/O workloads in mission-critical applications SATA drives with the lowest unit cost that provide a basic level of reliability and performance for non-mission-critical environments A device that functions as a switch to attach one or more initiators to one or more targets A device containing SSP, STP, and/or SMP initiator ports in a SAS domain 2