This article includes a list of references, related reading or external links, but its sources remain unclear because it lacks inline citations. Please improve this article by introducing more precise citations. (June 2010)

Serial Attached SCSI (SAS) is a communication protocol used to move data to and from computer storage devices such as hard drives and tape drives. SAS is a point-to-point serial protocol that replaces the parallel SCSI bus technology that first appeared in the mid 1980s in data centers and workstations, and it uses the standard SCSI command set. SAS offers backwards-compatibility with second-generation SATA drives. SATA 3 Gbit/s drives may be connected to SAS backplanes, but SAS drives may not be connected to SATA backplanes.

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[link]

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.

Identification and addressing[link]

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. It is assigned by the device manufacturer, like an Ethernet device's MAC address, and is typically world-wide 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 call a SAS address a World Wide Name or WWN, because it is essentially the same thing as a WWN in fibre channel.

Comparison with parallel SCSI[link]

This unreferenced section requires citations to ensure verifiability.

• The SAS bus operates 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 allows up to 65,535 devices through the use of expanders, while Parallel SCSI has a limit of 8 or 16 devices on a single channel.
• SAS allows a higher transfer speed (3 or 6 Gbit/s) than most parallel SCSI standards. SAS achieves these speeds on each initiator-target connection, hence getting higher throughput, whereas parallel SCSI shares the speed across the entire multidrop bus.
• SAS controllers may connect to SATA devices, either directly connected using native SATA protocol or through SAS expanders using SATA Tunneled Protocol (STP).
• Both SAS and parallel SCSI use the SCSI command-set.

Comparison with SATA[link]

This unreferenced section requires citations to ensure verifiability.

• Systems identify SATA devices by their port number connected to the host bus adapter, while SAS devices are uniquely identified by their World Wide Name (WWN).
• SAS protocol provides for multiple initiators in a SAS domain, while SATA has no analogous provision.
• Most SAS drives provide tagged command queuing, while most newer SATA drives provide native command queuing, each of which has its pros and cons.
• SATA uses the ATA command set; SAS uses the SCSI command set. Basic ATA has commands only for direct-access storage. However SCSI commands may be tunneled through ATA for devices such as CD/DVD drives.
• SAS hardware allows multipath I/O to devices while SATA (prior to SATA 3Gb/s) does not. Per specification, SATA 3Gb/s makes use of port multipliers to achieve port expansion. Some port multiplier manufacturers have implemented multipath I/O using port multiplier hardware.
• SATA is marketed as a general-purpose successor to parallel ATA and has become^[update] common in the consumer market, whereas the more-expensive SAS targets critical server applications.
• SAS error-recovery and error-reporting use SCSI commands which have more functionality than the ATA SMART commands used by SATA drives.
• SAS uses higher signaling voltages (800–1600 mV TX, 275–1600 mV RX) than SATA (400–600 mV TX, 325–600 mV RX). The higher voltage offers (among other features) the ability to use SAS in server backplanes.
• Because of its higher signaling voltages, SAS can use cables up to 10 m (33 ft) long, SATA has a cable-length limit of 1 m (3 ft) or 2 m (6.6 ft) for eSATA.

Characteristics[link]

Technical details[link]

The Serial Attached SCSI standard defines several layers (in order from highest to lowest):

• Application
• Transport
• Port
• Link
• PHY
• Physical

Serial Attached SCSI comprises three transport protocols:

• Serial SCSI Protocol (SSP) — for command-level communication with SCSI devices.
• Serial ATA Tunneling Protocol (STP) — for command-level communication with SATA devices.
• Serial Management Protocol (SMP) — for managing the SAS fabric.

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 present 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/SAS-1.1 adopted the physical signaling characteristics of SATA at the 1.5 Gbit/s and 3 Gbit/s rates, SAS-2.0 development of a 6 Gbit/s physical rate led the development of an equivalent SATA speed. According to the SCSI Trade Association, 12 Gbit/s is slated to follow 6 Gbit/s in a future SAS-3.0 specification.^[1]

Architecture[link]

Architecture of SAS layers

SAS architecture consists of six layers

• Physical layer:
  • defines electrical and physical characteristics
  • differential signaling transmission
  • Three connector types:
    • SFF 8482 – SATA compatible
    • SFF 8484 – up to four devices
    • SFF 8470 – external connector (InfiniBand connector), up to four devices
• PHY Layer:
  • 8b/10b data encoding
  • Link initialization, speed negotiation and reset sequences
  • Link capabilities negotiation (SAS-2)
• Link layer:
  • Insertion and deletion of primitives for clock-speed disparity matching
  • Primitive encoding
  • Data scrambling for reduced EMI
  • Establish and tear down native connections between SAS targets and initiators
  • Establish and tear down tunneled connections between SAS initiators and SATA targets connected to SAS expanders
  • Power management (proposed for SAS-2.1)
• Port layer:
  • Combining multiple PHYs with the same addresses into wide ports
• Transport layer:
  • Contains three transport protocols:
    • Serial SCSI Protocol (SSP): for command-level communication with SCSI devices
    • Serial ATA Tunneled Protocol (STP): for command-level communication with SATA devices
    • Serial Management Protocol (SMP): for managing the SAS fabric
• Application layer

Topology[link]

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[link]

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 allows multiple systems to be connected using a single switch port.

SAS 1 defined two different 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:

• An edge expander allows for communication with up to 255 SAS addresses, allowing the SAS initiator to communicate with these additional devices. Edge expanders can do direct table routing and subtractive routing. (For a brief discussion of these routing mechanisms, see below). Without a fanout expander, you can use at most two edge expanders in your delivery subsystem (because you will connect the subtractive routing port of those edge expanders together, and you can't connect any more expanders). To solve this bottleneck, you would use fanout expanders.

• A fanout expander can connect up to 255 sets of edge expanders, known as an edge expander device set, allowing for even more SAS devices to be addressed. The subtractive routing port of each edge expanders will be connected to the phys of fanout expander. A fanout expander cannot do subtractive routing, it can only forward subtractive routing requests to the connected edge expanders.

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 will pass 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 will 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 will not contain a single root node.

Connectors[link]

The SAS connector is much smaller than traditional parallel SCSI connectors, allowing for the small 2.5-inch (64 mm) drives. SAS currently provides for point data transfer speeds up to 6 Gbit/s, but is expected to reach 12 Gbit/s by the year 2012.^{[dated info]}

The physical SAS connector comes in several different variants:^[2]

Image	Codename	Other names	Ext./int.	No of pins	No of devices	Comment
	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)	Hi-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-8470	Infiniband CX4 connector, Molex LaneLink™	External	32	4	Hi-density external connector (also used as an internal connector).
	SFF-8086	Internal mini-SAS, internal mSAS	Internal	26	4	This is actually just a particular implementation of SFF-8086, which covers a variety of connectors, including SFF-8087 and SFF-8088 (below). This unnamed 26-circuit unshielded connector type is uncommon, as SFF-8087 is usually used instead.
	SFF-8087	Internal mini-SAS, internal mSAS, internal iSAS	Internal	36	4	Unshielded 36-circuit implementation of SFF-8086. Molex iPass™ reduced width internal 4× connector with future 10 Gbit/s capability.
	SFF-8088	External mini-SAS, external mSAS, external iSAS	External	26	4	Shielded 26-circuit implementation of SFF-8086. Molex iPass™ reduced width external 4× connector with future 10 Gbit/s capability.

Nearline SAS[link]

Nearline SAS or NL-SAS drives are enterprise SATA drives with a SAS interface, head, media, and rotational speed of traditional enterprise-class SATA drives with the fully capable SAS interface typical for classic SAS drives. System and storage vendors like Dell, EMC, Fujitsu, and IBM are offering these disks for SAN arrays, NAS solutions, and server systems.

They feature the following benefits compared to SATA:^[3]

• Dual ports allowing redundant paths
• Ability to connect a device to multiple computers
• Full SCSI command set
• Faster interface compared to SATA, up to 30%, no STP (Serial ATA Tunneling Protocol) overhead
• No need for SATA interposer cards (for high availability of SATA drives SATA interposer cards are needed)

To summarize: Nearline SAS drives are simply big, cheap, and slow SAS drives targeted toward nearline storage.

See also[link]

• List of device bandwidths
• USB Attached SCSI

References[link]

External links[link]

• T10 committee
• SCSI Trade Association
• Current draft revision of SAS 2 from T10 (6.83 MiB PDF after registration)
• Seagate whitepaper on Nearline SAS
• List of SAS controllers with connector type

v t e Computer bus official and de facto standards (wired) General System bus Front-side bus Back-side bus Daisy chain Control bus Address bus Bus contention Plug and play List of bus bandwidths Standards S-100 bus Unibus VAXBI MBus STD Bus SMBus Q-Bus ISA Zorro II Zorro III CAMAC FASTBUS LPC HP Precision Bus EISA VME VXI VXS NuBus TURBOchannel MCA SBus VLB PCI PXI HP GSC bus CoreConnect InfiniBand UPA PCI-X AGP PCI Express Intel QuickPath Interconnect HyperTransport Portable PC Card ExpressCard Embedded Multidrop bus AMBA Wishbone Storage ST-506 ESDI SMD Parallel ATA (PATA) DMA SSA HIPPI USB MSC FireWire (1394) Serial ATA (SATA) eSATA eSATAp SCSI Parallel SCSI Serial Attached SCSI (SAS) Fibre Channel (FC) iSCSI ATAoE Peripheral Apple Desktop Bus HIL MIDI Multibus RS-232 (serial port) DMX512-A IEEE-488 (GPIB) EIA/RS-422 IEEE-1284 (parallel port) UNI/O ACCESS.bus 1-Wire I²C SPI EIA/RS-485 Parallel SCSI Profibus USB FireWire (1394) Fibre Channel Camera Link External PCI Express x16 Thunderbolt Note: interfaces are listed in speed ascending order (roughly), the interface at the end of each section should be the fastest Category

v t e Solid-state drives Key terminology Encryption ECC Flash file system Flash memory - SLC/MLC Flash memory controller Garbage collection IOPS MB/s Over-provisioning Secure erase TRIM command Wear leveling Write amplification Flash manufacturers Hynix Intel Micron Samsung Toshiba Controllers Independent Indilinx JMicron Marvell Phison SandForce Captive Fusion-io Intel Samsung STEC SSD manufacturers List of solid-state drive manufacturers Interfaces SATA SAS FC USB PCIe NVM Express Related organizations INCITS JEDEC/JC-64.8 ONFI NVMHCI SATA-IO SFF Committee SNIA SSSI T10/SCSI T11/FC T13/ATA Category