Serial Attached SCSI
Encyclopedia
| Performance | Full-duplex with link aggregation Link aggregation Link aggregation or trunking or link bundling or Ethernet/network/NIC bonding or NIC teaming are computer networking umbrella terms to describe various methods of combining multiple network connections in parallel to increase throughput beyond what a single connection could sustain, and to provide... (4-ports wide at 24 Gbit/s) |
|---|---|
| 3.0 Gbit/s at introduction, 6.0 Gbit/s available February 2009 | |
| Connectivity | 10 m external cable |
| 255 device port expanders (>65k total devices) | |
| SAS-to-SATA compatibility | |
| Availability | Dual-port HDDs |
| Multi-initiator point-to-point | |
| Driver | Software-transparent with SCSI |
Serial Attached SCSI (SAS) is a computer bus
Computer bus
In computer architecture, a bus is a subsystem that transfers data between components inside a computer, or between computers.Early computer buses were literally parallel electrical wires with multiple connections, but the term is now used for any physical arrangement that provides the same...
used to move data to and from computer storage devices such as hard drives and tape drives. SAS depends on a point-to-point serial protocol that replaces the parallel SCSI
SCSI
Small Computer System Interface is a set of standards for physically connecting and transferring data between computers and peripheral devices. The SCSI standards define commands, protocols, and electrical and optical interfaces. SCSI is most commonly used for hard disks and tape drives, but it...
bus technology that first appeared in the mid 1980s in data centers and workstations, and it uses the standard SCSI command set
SCSI command
In SCSI computer storage, a command is the basic unit of communication. The SCSI command architecture was originally defined for parallel SCSI buses but has been carried forward with minimal change for use with Fibre Channel, iSCSI and Serial Attached SCSI....
. SAS offers backwards-compatibility with second-generation SATA
Serial ATA
Serial ATA is a computer bus interface for connecting host bus adapters to mass storage devices such as hard disk drives and optical drives...
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
SCSI Trade Association
The SCSI Trade Association, or SCSITA, is an industry trade group which exists to promote the use of SCSI technology. It was formed in 1996. , sponsor members include HP, Intel, LSI Logic, and Seagate...
(SCSITA) promotes the technology.
Introduction
A typical Serial Attached SCSI system consists of the following basic components:- An Initiator: a device that originates device-service and task-managementTask Management FunctionIn 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. Typically if a command is timed out this functionality is invoked to initiate a recovery for the...
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. - A Target: a device containing logical units and target ports that receives device service and task managementTask Management FunctionIn 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. Typically if a command is timed out this functionality is invoked to initiate a recovery for the...
requests for processing and sends responses for the same requests to initiator devices. A target device could be a hard diskHard diskA hard disk drive is a non-volatile, random access digital magnetic data storage device. It features rotating rigid platters on a motor-driven spindle within a protective enclosure. Data is magnetically read from and written to the platter by read/write heads that float on a film of air above the...
or a disk arrayDisk arrayA disk array is a disk storage system which contains multiple disk drives. It is differentiated from a disk enclosure, in that an array has cache memory and advanced functionality, like RAID and virtualization.Components of a typical disk array include:...
system. - A Service Delivery Subsystem: the part of an I/OInput/outputIn computing, input/output, or I/O, refers to the communication between an information processing system , and the outside world, possibly a human, or another information processing system. Inputs are the signals or data received by the system, and outputs are the signals or data sent from it...
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. - 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
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 EthernetEthernet
Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....
device's MAC address
MAC address
A Media Access Control address is a unique identifier assigned to network interfaces for communications on the physical network segment. MAC addresses are used for numerous network technologies and most IEEE 802 network technologies, including Ethernet...
, 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
- 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 terminationElectrical terminationElectrical termination of a signal involves providing a terminator at the end of a wire or cable to prevent an RF signal from being reflected back from the end, causing interference...
issues and does not require terminator packs like parallel SCSI. - SAS eliminates clock skewClock skew-In circuit design:In circuit designs, clock skew is a phenomenon in synchronous circuits in which the clock signal arrives at different components at different times...
. - SAS supports 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 supports 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 support connecting 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 SCSISCSISmall Computer System Interface is a set of standards for physically connecting and transferring data between computers and peripheral devices. The SCSI standards define commands, protocols, and electrical and optical interfaces. SCSI is most commonly used for hard disks and tape drives, but it...
command-set.
Comparison with SATA
- Systems identify SATA devices by their port number connected to the host bus adapter, while SAS devices are uniquely identified by their World Wide NameWorld Wide NameA World Wide Name or World Wide Identifier is a unique identifier which identifies a particular Fibre Channel, Advanced Technology Attachment or Serial Attached SCSI target...
(WWN). - SAS protocol supports multiple initiators in a SAS domain, while SATA has no analogous provision.
- Most SAS drives provide tagged command queuingTagged Command QueuingTagged Command Queuing is a technology built into certain ATA and SCSI hard drives. It allows the operating system to send multiple read and write requests to a hard drive. ATA TCQ is not identical in function to the more efficient native command queuing used by SATA drives...
, while most newer SATA drives provide native command queuingNative Command QueuingNative Command Queuing is a technology designed to increase performance of SATA hard disks under certain conditions by allowing the individual hard disk to internally optimize the order in which received read and write commands are executed...
, each of which has its pros and cons. - SATA uses the ATAAT AttachmentParallel ATA , originally ATA, is an interface standard for the connection of storage devices such as hard disks, solid-state drives, floppy drives, and optical disc drives in computers. The standard is maintained by X3/INCITS committee...
command set; SAS uses the SCSI command set. ATA directly supports only direct-access storage. However SCSI commands may be tunneled through ATA for devices such as CDCD-ROMA CD-ROM is a pre-pressed compact disc that contains data accessible to, but not writable by, a computer for data storage and music playback. The 1985 “Yellow Book” standard developed by Sony and Philips adapted the format to hold any form of binary data....
/DVD drives. - SAS hardware allows multipath I/OMultipath I/OIn computer storage, multipath I/O is a fault-tolerance and performance enhancement technique whereby there is more than one physical path between the CPU in a computer system and its mass storage devices through the buses, controllers, switches, and bridge devices connecting them.A simple example...
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 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 backplaneBackplaneA backplane is a group of connectors connected in parallel with 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. It is used as a backbone to connect several printed circuit boards together to make up a complete...
s. - Because of its higher signaling voltages, SAS can use cables up to 10 m (32.8 ft) long, SATA has a cable-length limit of 1 m (3.3 ft) or 2 m (6.6 ft) for eSATA.
Technical details
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) — supporting SAS disk drives.
- Serial ATA Tunneling Protocol (STP) — supporting SATA disks.
- Serial Management Protocol (SMP) — for managing SAS Expanders.
For the Link and PHY
PHY
PHY is an abbreviation for the physical layer of the OSI model.An instantiation of PHY connects a link layer device to a physical medium such as an optical fiber or copper cable. A PHY device typically includes a Physical Coding Sublayer and a Physical Medium Dependent layer. The PCS encodes and...
layers, SAS defines its own unique protocol.
At the physical layer
Physical layer
The physical layer or layer 1 is the first and lowest layer in the seven-layer OSI model of computer networking. The implementation of this layer is often termed PHY....
, 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 support 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.
Architecture
SAS architecture consists of six layers- Physical layer:
- defines electrical and physical characteristics
- differential signaling transmission
- Three connector types:
- SFFSmall Form Factor committeeThe Small Form Factor committee is an ad hoc electronics industry group formed to quickly develop interoperability specifications ....
8482 – SATASerial ATASerial ATA is a computer bus interface for connecting host bus adapters to mass storage devices such as hard disk drives and optical drives...
compatible - SFF 8484 – up to four devices
- SFF 8470 – external connector (InfiniBandInfiniBandInfiniBand is a switched fabric communications link used in high-performance computing and enterprise data centers. Its features include high throughput, low latency, quality of service and failover, and it is designed to be scalable...
connector), up to four devices
- SFF
- PHY Layer:
- 8b/10b data encoding8B/10B encodingIn telecommunications, 8b/10b is a line code that maps 8-bit symbols to 10-bit symbols to achieve DC-balance and bounded disparity, and yet provide enough state changes to allow reasonable clock recovery. This means that the difference between the count of 1s and 0s in a string of at least 20 bits...
- Link initialization, speed negotiation and reset sequences
- Link capabilities negotiation (SAS-2)
- 8b/10b data encoding
- Link layer:
- Insertion and deletion of primitives for clock-speed disparity matching
- Primitive encoding
- Data scrambling for reduced EMIElectromagnetic interferenceElectromagnetic interference is disturbance that affects an electrical circuit due to either electromagnetic induction or electromagnetic radiation emitted from an external source. The disturbance may interrupt, obstruct, or otherwise degrade or limit the effective performance of the circuit...
- 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:
- Supports three transport protocols:
- Serial SCSI Protocol (SSP): supports SAS devices
- Serial ATA Tunneled Protocol (STP): supports SATASerial ATASerial ATA is a computer bus interface for connecting host bus adapters to mass storage devices such as hard disk drives and optical drives...
devices attached to SAS expanders - Serial Management Protocol (SMP): provides for the configuration of SAS expanders
- Supports three transport protocols:
- Application layer
Topology
An initiatorSCSI initiator
In computer data storage, a SCSI initiator is the endpoint that initiates a SCSI session, that is, sends a SCSI command. The initiator usually does not provide any Logical Unit Numbers ....
may connect directly to a target via one or more PHY
PHY
PHY is an abbreviation for the physical layer of the OSI model.An instantiation of PHY connects a link layer device to a physical medium such as an optical fiber or copper cable. A PHY device typically includes a Physical Coding Sublayer and a Physical Medium Dependent layer. The PCS encodes and...
s (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 switchNetwork switch
A network switch or switching hub is a computer networking device that connects network segments.The term commonly refers to a multi-port network bridge that processes and routes data at the data link layer of the OSI model...
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
The SAS connector is much smaller than traditional parallel SCSI connectorSCSI connector
A SCSI connector is used to connect together computer parts that use a system called SCSI to communicate with each other. Generally, two connectors, designated male and female, plug together to form a connection which allows two components, such as a computer and a disk drive, to communicate with...
s, allowing for the small 2.5 inches (63.5 mm) drives. SAS currently supports point data transfer speeds up to 6 Gbit/s, but is expected to reach 12 Gbit/s by the year 2012.
The physical SAS connector comes in several different variants:
| 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 SGPIO SGPIO is an acronym for Serial General Purpose Input/Output which is a 4-signal bus used between a host bus adapter and a backplane; of the 4 signals, 3 are driven by the HBA and 1 by the backplane... (extension of SFF 8484), a serial link protocol used usually for LED indicators. |
|||||
| SFF-8470 | Infiniband InfiniBand InfiniBand is a switched fabric communications link used in high-performance computing and enterprise data centers. Its features include high throughput, low latency, quality of service and failover, and it is designed to be scalable... 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 | Note: very similar to the SFF-8087 (below) but less common. | |
| SFF-8087 | Internal mini-SAS, internal mSAS | Internal | 36 | 4 | Molex Molex connector Molex connector is the vernacular term for a two-piece pin and socket interconnection, most frequently disk drive connectors. Pioneered by Molex Connector Company, the two-piece design became an early electronic standard. Molex developed and patented the first examples of this connector style in... iPass™ reduced width internal 4× connector with future 10 Gbit/s support. |
|
| SFF-8088 | External mini-SAS, external mSAS | External | 26 | 4 | Molex Molex connector Molex connector is the vernacular term for a two-piece pin and socket interconnection, most frequently disk drive connectors. Pioneered by Molex Connector Company, the two-piece design became an early electronic standard. Molex developed and patented the first examples of this connector style in... iPass™ reduced width external 4× connector with future 10 Gbit/s support. |
Nearline SAS
NearlineNearline storage
Nearline storage is a term used in computer science to describe an intermediate type of data storage that represents a compromise between online storage and offline storage/archiving...
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:
- Dual ports allowing redundant paths
- Multiple host support
- 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.
External links
- T10 committee
- SCSI Trade Association
- Current draft revision of SAS 2 from T10 (6.83 MiB PDF after registration)
- Seagate whitepaper on Nearline SAS