Serial Attached SCSI

SAS
Serial Attached SCSI

Four red cables lead into a wide black electrical connector

SAS connector
Width in bits 1
Number of devices 65,535
Speed
  • SAS-1: 3 Gbit/s (2005)
  • SAS-2: 6 Gbit/s (2009)
  • SAS-3: 12 Gbit/s (2013)
  • SAS-4: 22.5 Gbit/s (future)
Style Serial

Serial Attached SCSI (SAS) is a point-to-point serial protocol that moves data to and from computer storage devices such as hard drives and tape drives. SAS replaces the older Parallel SCSI (Small Computer System Interface, usually pronounced "scuzzy") bus technology that first appeared in the mid-1980s. SAS, like its predecessor, uses the standard SCSI command set. SAS offers backward compatibility with SATA, versions 2 and later. This allows for SATA drives to be connected to SAS backplanes. The reverse, connecting SAS drives to SATA backplanes, is not possible.[1]

The T10 technical committee of the International Committee for Information Technology Standards (INCITS) develops and maintains the SAS protocol; the SCSI Trade Association (SCSITA) promotes the technology.

Introduction

Storage servers housing 24 SAS hard disk drives per server

A typical Serial Attached SCSI system consists of the following basic components:

  1. An initiator: a device that originates device-service and task-management requests for processing by a target device and receives responses for the same requests from other target devices. Initiators may be provided as an on-board component on the motherboard (as is the case with many server-oriented motherboards) or as an add-on host bus adapter.
  2. A target: a device containing logical units and target ports that receives device service and task management requests for processing and sends responses for the same requests to initiator devices. A target device could be a hard disk or a disk array system.
  3. A service delivery subsystem: the part of an I/O system that transmits information between an initiator and a target. Typically cables connecting an initiator and target with or without expanders and backplanes constitute a service delivery subsystem.
  4. Expanders: devices that form part of a service delivery subsystem and facilitate communication between SAS devices. Expanders facilitate the connection of multiple SAS End devices to a single initiator port.[2]

History

Identification and addressing

A SAS Domain is the SAS version of a SCSI domain—it consists of a set of SAS devices that communicate with one another by means of a service delivery subsystem. Each SAS port in a SAS domain has a SCSI port identifier that identifies the port uniquely within the SAS domain, the World Wide Name. It is assigned by the device manufacturer, like an Ethernet device's MAC address, and is typically worldwide unique as well. SAS devices use these port identifiers to address communications to each other.

In addition, every SAS device has a SCSI device name, which identifies the SAS device uniquely in the world. One doesn't often see these device names because the port identifiers tend to identify the device sufficiently.

For comparison, in parallel SCSI, the SCSI ID is the port identifier and device name. In Fibre Channel, the port identifier is a WWPN and the device name is a WWNN.

In SAS, both SCSI port identifiers and SCSI device names take the form of a SAS address, which is a 64 bit value, normally in the NAA IEEE Registered format. People sometimes refer to a SCSI port identifier as the SAS address of a device, out of confusion. People sometimes call a SAS address a World Wide Name or WWN, because it is essentially the same thing as a WWN in Fibre Channel. For a SAS expander device, the SCSI port identifier and SCSI device name are the same SAS address.

Comparison with parallel SCSI

Comparison with SATA

There is little physical difference between SAS and SATA.[6]

Characteristics

Technical details

The Serial Attached SCSI standard defines several layers (in order from highest to lowest): application, transport, port, link, PHY and physical. Serial Attached SCSI comprises three transport protocols:

For the Link and PHY layers, SAS defines its own unique protocol.

At the physical layer, the SAS standard defines connectors and voltage levels. The physical characteristics of the SAS wiring and signaling are compatible with and have loosely tracked that of SATA up to the 6 Gbit/s rate, although SAS defines more rigorous physical signaling specifications as well as a wider allowable differential voltage swing intended to allow longer cabling. While SAS-1.0 and SAS-1.1 adopted the physical signaling characteristics of SATA at the 3 Gbit/s rate with 8b/10b encoding, SAS-2.0 development of a 6 Gbit/s physical rate led the development of an equivalent SATA speed. In 2013, 12 Gbit/s followed in the SAS-3 specification.[8] SAS-4 is slated to introduce 22.5 Gbit/s signaling with a more efficient 128b/150b encoding scheme to realize a usable data rate of 2,400 MB/s while retaining compatibility with 6 and 12 Gbit/s.[9]

Additionally, SCSI Express takes advantage of the PCI Express infrastructure to directly connect SCSI devices over a more universal interface.[10]

Architecture

The architecture of SAS layers

SAS architecture consists of six layers:

Topology

An initiator may connect directly to a target via one or more PHYs (such a connection is called a port whether it uses one or more PHYs, although the term wide port is sometimes used for a multi-PHY connection).

SAS expanders

The components known as Serial Attached SCSI Expanders (SAS Expanders) facilitate communication between large numbers of SAS devices. Expanders contain two or more external expander-ports. Each expander device contains at least one SAS Management Protocol target port for management and may contain SAS devices itself. For example, an expander may include a Serial SCSI Protocol target port for access to a peripheral device. An expander is not necessary to interface a SAS initiator and target but allows a single initiator to communicate with more SAS/SATA targets. A useful analogy: one can regard an expander as akin to a network switch in a network, which connects multiple systems using a single switch port.

SAS 1 defined two types of expander; however, the SAS-2.0 standard has dropped the distinction between the two, as it created unnecessary topological limitations with no realized benefit:

Direct routing allows a device to identify devices directly connected to it. Table routing identifies devices connected to the expanders connected to a device's own PHY. Subtractive routing is used when you are not able to find the devices in the sub-branch you belong to. This passes the request to a different branch altogether.

Expanders exist to allow more complex interconnect topologies. Expanders assist in link-switching (as opposed to packet-switching) end-devices (initiators or targets). They may locate an end-device either directly (when the end-device is connected to it), via a routing table (a mapping of end-device IDs and the expander the link should be switched to downstream to route towards that ID), or when those methods fail, via subtractive routing: the link is routed to a single expander connected to a subtractive routing port. If there is no expander connected to a subtractive port, the end-device cannot be reached.

Expanders with no PHYs configured as subtractive act as fanout expanders and can connect to any number of other expanders. Expanders with subtractive PHYs may only connect to two other expanders at a maximum, and in that case they must connect to one expander via a subtractive port and the other via a non-subtractive port.

SAS-1.1 topologies built with expanders generally contain one root node in a SAS domain with the one exception case being topologies that contain two expanders connected via a subtractive-to-subtractive port. If it exists, the root node is the expander, which is not connected to another expander via a subtractive port. Therefore, if a fanout expander exists in the configuration, it must be the domain's root node. The root node contains routes for all end devices connected to the domain. Note that with the advent in SAS-2.0 of table-to-table routing and new rules for end-to-end zoning, more complex topologies built upon SAS-2.0 rules do not contain a single root node.

Connectors

The SAS connector is much smaller than traditional parallel SCSI connectors, allowing for the small 2.5-inch (64 mm) drives. Commonly, SAS provides for point data transfer speeds up to 6 Gbit/s, but 12 Gbit/s products have begun shipping in 2013.[12]

The physical SAS connector comes in several different variants:[13]

Image Codename Other names Ext./int. Pins No of devices Comment
SFF-8086 Internal mini-SAS, internal mSAS Internal 26 4 This is a less common implementation of SFF-8087 than the 36-circuit version. The fewer positions is enabled by it not supporting sidebands.
SFF-8087 Internal mini-SAS, internal mSAS, internal iSAS, internal iPass Internal 36 4 Unshielded 36-circuit implementation of SFF-8086. Molex iPass reduced width internal 4× connector, 12 Gbit/s capability.
SFF-8088 External mini-SAS, external mSAS, external iSAS, external iPass External 26 4 Shielded 26-circuit implementation of SFF-8086. Molex iPass reduced width external 4× connector, 12 Gbit/s capability.
SFF-8470 InfiniBand CX4 connector, Molex LaneLink External 34 4 High-density external connector (also used as an internal connector).
SFF-8482 Internal 29 1 This form factor is designed for compatibility with SATA. The socket is compatible with SATA drives; however, the SATA socket is not compatible with SFF-8482 (SAS) drives. The pictured connector is a drive-side connector.
SFF-8484 Internal 32 (19) 4 (2) High-density internal connector, 2 and 4 lane versions are defined by the SFF standard.
SFF-8485 Defines SGPIO (extension of SFF 8484), a serial link protocol used usually for LED indicators.
SFF-8643 Internal 36 4/8 Mini-SAS HD (introduced with SAS 12 Gbit/s)
SFF-8644 External 4/8 Mini-SAS HD (introduced with SAS 12 Gbit/s)
Sideband connector Internal Often seen on 1xSFF-8643 > 4xSATA cables. Connects the controller to the backplane or disk enclosure if exist. Connection is optional.
SFF-8680 Internal 1 (2 ports) SAS 12 Gbit/s backplane connector
SFF-8639 / U.2 [14] (revision of the SFF-8680) Internal 68 1 (2 ports) SAS 12 Gbit/s backplane connector

Nearline SAS

Nearline SAS (abbreviated to NL-SAS, and sometimes called midline SAS) drives have a SAS interface, but head, media, and rotational speed of traditional enterprise-class SATA drives, so they cost less than other SAS drives. When compared to SATA, NL-SAS drives have the following benefits:[15]:20

See also

References

  1. ↑ "SAS and SATA: Unparalleled Compatibility". Retrieved 2013-08-05.
  2. ↑ "SAS architecture". ibm. Retrieved January 14, 2016.
  3. 1 2 "Serial Attached SCSI Master Roadmap". SCSI Trade Association. 2015-10-14. Retrieved 2016-02-26.
  4. ↑ "Serial Attached SCSI - 4 (SAS-4) draft" (PDF). T10. 2016-05-11. Retrieved 2016-05-15.
  5. ↑ "Redundancy in enterprise storage networks using dual-domain SAS configurations". Hewlett-Packard Development Company. May 2008. Archived from the original (PDF) on 2016-01-10. Retrieved 2016-01-10.
  6. 1 2 3 4 5 6 7 8 Steadfast. "SATA vs SAS Hard Drives on Dedicated Servers". Retrieved 2013-08-05.
  7. ↑ Patrick Schmid; Achim Roos (2009-08-31). "SAS Features And Basics - Next-Generation SAS: 6 Gb/s Storage Hits The Enterprise". Tom's Hardware. Retrieved 2014-07-15.
  8. ↑ "Serial Attached SCSI - 3 (SAS-3)" (PDF). T10. 2013-11-07. Retrieved 2015-05-11.
  9. ↑ "Serial Attached SCSI - 4 (SAS-4), 5.8.1 General electrical characteristics" (PDF). Retrieved 2015-05-11.
  10. ↑ "Library » SCSI Express". SCSI Trade Association. Retrieved 2013-08-05.
  11. ↑ "SAS Protocol Layer - 4 (SPL-4) draft, p." (PDF). T10. 2016-05-09. Retrieved 2016-05-15.
  12. ↑ "LSI First to Ship New High-Performance 12Gb/s SAS Products". Retrieved 2013-12-03.
  13. ↑ "SFF Committee specifications". Retrieved 2013-08-05.
  14. ↑ "PCPer SFF-8639 Review". Retrieved 2016-07-21.
  15. 1 2 3 Willis Whittington (2007). "Desktop, Nearline & Enterprise Disk Drives" (PDF). Storage Networking Industry Association (SNIA). Retrieved 2014-09-22.
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