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Serial Attached SCSI

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Serial Attached SCSI (SAS) is a serial communication protocol for computer storage devices. It is designed for the corporate and enterprise market as a replacement for SCSI, allowing for much higher speed data transfers than previously available, and is backwards-compatible with SATA. As the name suggests, SAS uses serial communication instead of the parallel method found in traditional SCSI devices, but still uses SCSI commands for interacting with SAS devices.

Introduction

The Serial Attached SCSI system consists of 3 basic components:

  • Initiators (the hosts which will use the system),
  • Expanders, and
  • Targets (disk arrays).

Initiators may be provided as an on board component of the motherboard (as is the case with newer SuperMicro brand motherboards) or through the use of an add-on host bus adapter.

The sum of all devices used in an SAS implementation is known as the "SAS Domain". This is merely a term to refer to the network of devices and their corresponding "World Wide Names" (WWN). The World Wide Name is a globally unique identifier assigned to a device assigned by the manufacturer (similar to that of an Ethernet device's MAC address). An SAS domain may have a total of 16,256 devices.

Technical details

SAS supports up to 16,384 addressable devices in an SAS domain and point to point data transfer speeds up to 3 Gbit/s, but is expected to reach 10 Gbit/s by the year 2010. The SAS connector is much smaller than traditional parallel SCSI connectors allowing for small 2.5 inch drives.

The physical SAS connector is available in 3 different variants:

  • SFF 8482 — which is form factor compatible with SATA,
  • SFF 8484 — hi-density internal connector for connecting up to 4 devices,
  • SFF 8470 — hi-density external connector (aka Infiniband connector) for connecting up to 4 devices.

Form factor compatibility with SATA allows for much cheaper SATA drives to connect to an SAS backplane. SAS drives are not compatible on a SATA bus and have their physical connector keyed to prevent any plugging into a SATA backplane.

Serial Attached SCSI supports three transport protocols:

  • Serial SCSI Protocol (SSP) — Supporting SAS disk drives.
  • Serial ATA Tunneling Protocol (STP) — Supporting SATA disks.
  • Serial Management Protocol (SMP) — for managing SAS Expanders.

An SAS domain is a set of SAS ports communicating with each other. An SAS domain contains one or more SAS devices and a service delivery sub-system. An SAS domain may be a SCSI domain. Each SAS device is assigned a World Wide Name (aka SAS address) assigned by IEEE for the particular vendor. The WWN uniquely identifies the device in an SAS domain just as a SCSI ID identifies a device in a parallel SCSI bus.

Topology

Within a complex SAS domain, initiators are connected to edge expanders. Edge expanders may be connected to targets or a fanout expander. Fanout expanders may be connected to any combination of initiators, edge expanders, and targets. Each expander (Edge or Fanout) may connect to 128 other devices, though edge expanders may only connect to one other expander. Fanout expanders may connect any number of other edge expanders up to the 128 device limit. Another important note is that there may only be one Fanout expander within an SAS domain.

See also

External references

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