Serial Attached SCSI

Technical specificationsSerial Attached SCSI
PerformanceFull-duplex with link aggregation (wide ports at 24 Gbit/s)
3.0 Gbit/s at introduction, 6.0 Gbit/s,

12.0 Gbit/s,

22.2 Gbit/s planned

Connectivity8 meter external cable
128 device port expanders (16K + total devices)
SAS-to-SATA compatibility
AvailabilityDual-port HDDs
Multi-initiator point-to-point
DriverSoftware-transparent with SCSI

Serial Attached SCSI (SAS) is a technology designed to move data to and from computer storage devices such as hard drives and tape drives. It is a point-to-point serial protocol that replaces the parallel SCSI. SCSI first appeared in the mid 1980s in corporate data centers. SAS uses the standard SCSI command set. At present it is slightly slower than the final parallel SCSI implementation, but in 2009 it will double its present speed to 6 Gbit/s. This will permit much higher speed data transfers. The protocol is "downwards"-compatible with second generation SATA drives. These drives may be connected to SAS backplanes (controllers), but SAS drives can not be connected to SATA backplanes.

The SAS protocol is developed and maintained by the T10 technical committee of the International Committee for Information Technology Standards (INCITS) and promoted by the SCSI Trade Association (SCSITA).

SAS (Serial Attached SCSI) vs parallel SCSI

  • The SAS bus is point-to-point while the SCSI bus is multidrop. Each SAS device is connected by a dedicated link to the initiator, unless an expander is used. If one initiator is connected to one target, there is no opportunity for contention; with parallel SCSI, even this situation could cause contention.
  • SAS has no termination issues and does not require terminator packs like parallel SCSI.
  • SAS eliminates clock skew.
  • SAS supports up to 16,384 devices through the use of expanders while Parallel SCSI is limited to 8, 16, or 32 devices (including the SCSI controller) on a single channel.
  • SAS supports a higher transfer speed (1.5 OR 3.0 Gbit/s) than most parallel SCSI standards. The speed is realized on each initiator-target connection, hence higher throughput whereas in parallel SCSI the speed is shared across the entire multidrop bus.
  • SAS controllers are required by the standard to support SATA devices.
  • Both SAS and parallel SCSI use the SCSI command-set.
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