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2.5" and 3.5" SAS Hard Drives (A Hershey's Kiss for size comparison.)

Serial Attached SCSI (SAS) is a computer bus technology primarily designed for transfer of data to and from devices like hard drives, CD-ROM drives and so on. SAS is a serial communication protocol for direct attached storage (DAS) devices. It is designed for the corporate and enterprise market as a replacement for parallel SCSI, allowing for much higher speed data transfers than previously available, and is backwards-compatible with SATA drives. Though SAS uses serial communication instead of the parallel method found in traditional SCSI devices, it still uses SCSI commands for interacting with SAS End devices. Image File history File links Broom_icon. ... Image File history File linksMetadata Size of this preview: 450 × 600 pixelsFull resolution (2112 × 2816 pixel, file size: 1. ... Image File history File linksMetadata Size of this preview: 450 × 600 pixelsFull resolution (2112 × 2816 pixel, file size: 1. ... Hersheys Kisses filled with Caramel. ... In computer architecture, a bus is a subsystem that transfers data or power between computer components inside a computer or between computers and typically is controlled by device driver software. ... Typical hard drives of the mid-1990s. ... The CD-ROM (an abbreviation for Compact Disc Read-Only Memory (ROM)) is a non-volatile optical data storage medium using the same physical format as audio compact discs, readable by a computer with a CD-ROM drive. ... Direct Attached Storage (DAS) refers to a digital storage system directly attached to a server or workstation, without a storage network in between. ... Scuzzy redirects here. ... SATA redirects here. ... In telecommunications and computer science, serial communications is the process of sending data one bit at one time, sequentially, over a communications channel or computer bus. ... This article is about the Centronics style port. ... Scuzzy redirects here. ... In SCSI computer storage, a command is the basic unit of communication. ...

The SAS protocol is developed and maintained by the T10 committee. The current draft revision of SAS protocol can be downloaded from SAS 2 draftPDF (6.47 MiB). “PDF” redirects here. ... MiB redirects here. ...

Introduction

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

1. An Initiator is a device that originates device service and task management requests to be processed 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 is 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'..... is 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 are devices that are part of a service delivery subsystem and facilitate communication between SAS devices. It facilitates connection of multiple SAS End devices to a single initiator port.

In computer storage software, a task management function is an error recovery mechanism implemented by the software to influence and alter processing of certain commands, their sequence and so on. ... In general terms, a Host Bus Adapter (HBA) is any adapter that allows a computer bus to attach to another bus or channel. ... In computer storage software, a task management function is an error recovery mechanism implemented by the software to influence and alter processing of certain commands, their sequence and so on. ... Typical hard drives of the mid-1990s. ... Hewlett-Packard Disk-Arrays: HASS (top) and NIKE (OEMd Data General SCSI Clariion) EMC CLARiiON CX500 (Cover removed on one Shelf) EMC Symmetrix DMX1000 A disk array is an enterprise storage system which contains multiple disk drives. ... Energy Input: The energy placed into a reaction. ...

SAS Domain & WWN (World Wide Name)

A "SAS Domain" is an I/O system consisting of a set of SAS devices that communicate with one another by means of a service delivery subsystem. Each SAS device in a SAS domain has a globally unique identifier assigned to the device manufacturer by IEEE (similar to an Ethernet device's MAC address) called a World Wide Name (aka SAS address). The WWN uniquely identifies the device in the SAS domain just as a SCSI ID identifies a device in a parallel SCSI bus. A SAS domain may contain up to a total of 16,256 devices. Energy Input: The energy placed into a reaction. ... The Institute of Electrical and Electronics Engineers or IEEE (pronounced as eye-triple-ee) is an international non-profit, professional organization incorporated in the State of New York, United States. ... Ethernet is a large, diverse family of frame-based computer networking technologies that operate at many speeds for local area networks (LANs). ... In computer networking a Media Access Control address (MAC address) or Ethernet Hardware Address (EHA) or hardware address or adapter address is a quasi-unique identifier attached to most network adapters (NICs). ... A World Wide Name (WWN) or World Wide Identifier (WWID) is a unique identifier in a Fibre Channel or Serial Attached SCSI storage network. ...

SAS (Serial Attached SCSI) vs parallel SCSI

• The SAS bus is point-to-point while the SCSI bus is multidrop. Each 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 on a single channel.
• SAS supports a higher transfer speed (1.5, 3.0 or 6.0 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 typically support SATA devices.^{[citation needed]}
• SAS uses the SCSI command-set.

Point-to-Point telecommunications is most recently (2003) referenced regarding wireless data communications for Internet or Voice over IP via radio frequencies in the multi-gigahertz range. ... This article needs to be cleaned up to conform to a higher standard of quality. ... Electrical termination of a signal ceases its energy flow through the electrical circuit. ... // In circuit design, clock skew (sometimes timing skew) is a phenomenon in synchronous circuits in which the clock signal (sent from the clock circuit) arrives at different components at different times. ... A gigabit per second (gbps or gbit/s) is a unit of data transmission equal to 1,000 megabits per second or 1,000,000 kilobits per second or 1,000,000,000 bits per second. ... This article needs to be cleaned up to conform to a higher standard of quality. ... Scuzzy redirects here. ...

SAS vs SATA

• SATA devices are uniquely identified by their port number connected to the Host bus adapter while SAS devices are uniquely identifed by their World Wide Name (WWN).
• 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 follows the ATA command set and thus only supports hard drives and CD/DVD drives. In theory, SAS also supports numerous other devices including scanners and printers. However, this advantage could also be moot, as most such devices have also found alternative paths via such buses as USB, IEEE 1394 (FireWire), and Ethernet.
• SAS hardware allows multipath I/O to devices while SATA does not. An effort is underway to use port multipliers to achieve multipathing with SATA 3.0 Gbit/s.
• SATA is marketed as a general-purpose successor to Parallel ATA and is now common in the consumer market, while the more expensive SAS is marketed for critical server applications.
• SAS error recovery and reporting are much cleaner ^[clarify] than SATA.
• SAS uses higher signaling voltages (800-1600 mV TX, 275-1600 mV RX) than SATA (400-600 mV TX, 325-600 mV RX). When SAS is mixed with SATA, the SAS drives run at SATA-voltages. One reason for this higher voltage is so SAS may be used in server backplanes.
• Because of its higher signaling voltages, SAS can use cables up to 8 m (25 ft) long, SATA is limited to 1 m (3 ft).

In general terms, a Host Bus Adapter (HBA) is any adapter that allows a computer bus to attach to another bus or channel. ... A World Wide Name (WWN) or World Wide Identifier (WWID) is a unique identifier in a Fibre Channel or Serial Attached SCSI storage network. ... TCQ redirects here; it is also the IATA code for the Crnl. ... Native Command Queuing (NCQ) is a technology designed to increase performance of SATA hard disks by allowing the individual hard disk to receive more than one I/O request at a time and dynamically change the order in which they are applied. ... USB redirects here. ... A 6-Pin Firewire 400 connector FireWire (also known as i. ... Ethernet is a large, diverse family of frame-based computer networking technologies that operate at many speeds for local area networks (LANs). ... In computer storage, multipath I/O is an arrangement whereby there is more than one logical path between the CPU in a computer system and its mass storage devices through the buses and bridge devices connecting them. ... ATA cables: 40 wire ribbon cable top, 80 wire ribbon cable bottom Advanced Technology Attachment (ATA), is a standard interface for connecting storage devices such as hard disks and CD-ROM drives inside personal computers. ... A backplane is a circuit board (usually a printed circuit board) that connects several connectors in parallel to each other, so that each pin of each connector is linked to the same relative pin of all the other connectors, forming a computer bus. ...

SAS Expanders

A Serial Attached SCSI Expander (SAS Expander) is a component used to 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 an SAS initiator and target but, if connected, helps a single initiator to communicate with more SAS/SATA targets. A useful analogy: an expander can be considered akin to an ethernet hub in a network which allows multiple systems to be connected using a single switch port. Bold text 4 port ethernet hub An Ethernet hub or concentrator is a device for connecting multiple twisted pair or fiber optic Ethernet devices together, making them act as a single segment. ...

There are two different types of expander: Edge Expanders and Fanout Expanders.

• An edge expander allows for communication with up to 128 SAS addresses, allowing the SAS initiator to communicate with these additional devices. Edge expanders are the ones which can do direct table routing and subtractive routing. A brief discussion of these routing mechanisms is 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 128 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 can not do subtractive routing, it can only forward subtractive routing requests to the connected edge expanders.

• A dual expander is an expander (either fanout or edge) featuring redundant links.

Direct routing allows a device to identify devices directly connected to it. Table routing is for identifying 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.

Connectors

The SAS connector is much smaller than traditional parallel SCSI connectors allowing for the small 2.5 inch drives. SAS supports point data transfer speeds up to 3 Gbit/s, but is expected to reach 12 Gbit/s by the year 2012. There are many different types of SCSI connector in use in the computer industry. ... A gigabit per second (Gbps or Gbit/s) is a unit of data transmission equal to 1,000 megabits per second or 1,000,000 kilobits per second or 1,000,000,000 bits per second. ... 2012 (MMXII) will be a leap year starting on Sunday of the Gregorian calendar. ...

The physical SAS connector is available in several different variants:

Image	Codename	Also known as	Ext/int	# of pins	# of devices	Comment
	SFF 8482	SATA connector	Internal		1	Form factor compatible with SATA: allows for SATA drives to connect to a SAS backplane, which obviates the need to install an additional SATA controller just to attach a DVD-writer, for example. Note that SAS drives are not usable on a SATA bus and have their physical connector keyed to prevent any plugging into a SATA backplane.
	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 connector	External	32	4	Hi-density external connector (also used as an internal connector)
	SFF 8087	Internal mini-SAS	Internal		4	Molex iPASS reduced width internal 4x connector with future 10 Gbit/s support
	SFF 8088	External mini-SAS	External	32	4	Molex iPASS reduced width external 4x connector with future 10 Gbit/s support

A backplane is a circuit board (usually a printed circuit board) that connects several connectors in parallel to each other, so that each pin of each connector is linked to the same relative pin of all the other connectors, forming a computer bus. ... Image File history File links Metadata Size of this preview: 800 × 600 pixelsFull resolution‎ (3,264 × 2,448 pixels, file size: 2. ... Image File history File links Metadata Size of this preview: 800 × 600 pixelsFull resolution‎ (1,280 × 960 pixels, file size: 318 KB, MIME type: image/jpeg) File historyClick on a date/time to view the file as it appeared at that time. ... InfiniBand is a switched fabric communications link primarily used in high-performance computing. ... Image File history File links Metadata Size of this preview: 800 × 600 pixelsFull resolution‎ (2,304 × 1,728 pixels, file size: 980 KB, MIME type: image/jpeg) File historyClick on a date/time to view the file as it appeared at that time. ... The 4-pin power connector used in PCs commonly known as a Molex connector. ... Image File history File links Metadata Size of this preview: 800 × 600 pixelsFull resolution‎ (1,600 × 1,200 pixels, file size: 539 KB, MIME type: image/jpeg) File historyClick on a date/time to view the file as it appeared at that time. ... The 4-pin power connector used in PCs commonly known as a Molex connector. ...

Technical details

Serial Attached SCSI is comprised of 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.

Topology

An initiator may be directly connected 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). Another meaning of initiator is SCSI initiator. ... In SCSI computer storage, a target is the storage-device side endpoint of a SCSI session. ... PHY is a generic electronics term refering to a special electronic integrated circuit or functional block of a circuit that provides physical access to a digital connection cable. ...

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.

There exists one root (most 'upstream') node in a SAS domain. This 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 knows about all end devices connected to the domain.

See also

• List of device bandwidths

This is a list of device bandwidths: the channel capacity (or, more informally, bandwidth) of some computer devices employing methods of data transport is listed by bit/s, kilobit/s (kbit/s), megabit/s (Mbit/s), or gigabit/s (Gbit/s) as appropriate and also MB/s or megabytes per...

External references

• T10 committee
• Download SAS 2 draft revision from T10PDF (6.7 MiB)
• Implementing SAS storage