Serial attached SCSI

http://en.wikipedia.org/wiki/Serial_attached_SCSI

Serial attached SCSI

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SAS

Serial attached SCSI
SAS connector
Width in bits	1
Number of devices	65,535
Speed	3.0 Gbit/s at introduction, 6.0 Gbit/s available February 2009, 12.0 Gbit/s in development
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, 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 second-generation SATA drives. SATA 3 or 6 Gbit/s drives may be connected to SAS backplanes, but SAS drives cannot connect to SATA backplanes.^[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.

Contents

[hide] 

• 1 Introduction
• 2 Identification and addressing
• 3 Comparison with parallel SCSI
• 4 Comparison with SATA
• 5 Characteristics
  • 5.1 Technical details
  • 5.2 Architecture
  • 5.3 Topology
  • 5.4 SAS expanders
  • 5.5 Connectors
• 6 Nearline SAS
• 7 See also
• 8 References
• 9 External links

Introduction[edit]

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 andbackplanes 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[edit]

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 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[edit]

This section does not cite any references or sources. Please help improve this section by adding citations to reliable sources. Unsourced material may be challenged and removed. (July 2009)

• 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 devices feature dual ports, allowing for redundant backplanes/multipath I/O.
• 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[edit]

There is little physical difference between SAS and SATA.^[2]

• Systems identify SATA devices by their port number connected to the host bus adapter or by their Universally unique identifier (UUID), 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.^[2]
• Most SAS drives provide tagged command queuing, while most newer SATA drives provide native command queuing,^[2] each of which has its pros and cons.
• SATA uses a command set that is based on the parallel ATA command set and then extended beyond that set to include features like native command queuing, hot-plugging, and TRIM. SAS uses the SCSI command set, which includes a wider range of features like error recovery, reservations and block reclamation. Basic ATA has commands only for direct-access storage. However SCSI commands may be tunneled through ATAPI^[2] for devices such as CD/DVD drives.
• SAS hardware allows multipath I/O to devices while SATA (prior to SATA 3Gb/s) does not.^[2] 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 common in the consumer market, whereas the more-expensive SAS targets critical server applications.
• SAS error-recovery and error-reporting uses SCSI commands, which have more functionality than the ATA SMART commands used by SATA drives.^[2]
• 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 serverbackplanes.^[2]
• Because of its higher signaling voltages, SAS can use cables up to 10 m (33 ft) long, whereas SATA has a cable-length limit of 1 m (3.3 ft) or 2 m (6.6 ft) for eSATA.^[2]

Characteristics[edit]

Technical details[edit]

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 2013 SAS-3.0 specification.^[3][4][5] Additionally, SCSI Express takes advantage of PCI Express infrastructure to directly connect SCSI devices over the more universal interface.^[6]

Architecture[edit]

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[edit]

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[edit]

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 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 a delivery subsystem (because you connect the subtractive routing port of those edge expanders together, and you can't connect any more expanders). Fanout expanders solve this bottleneck.

• A fanout expander can connect up to 255 sets of edge expanders, known as an edge expander device set, letting even more SAS devices be addressed. The subtractive routing port of each edge expanders connects 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 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[edit]

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:^[7]

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)	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-8470	InfiniBand CX4 connector, Molex LaneLink	External	32	4	High-density external connector (also used as an internal connector).
	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 with future 10 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 with future 10 Gbit/s capability.

Nearline SAS[edit]

Nearline SAS or NL-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.

They have the following benefits compared to SATA:^[8]

• Dual ports allowing redundant paths
• Ability to connect a device to multiple computers
• Full SCSI command set
• Faster interface compared to SATA, up to 20%, no STP (Serial ATA Tunneling Protocol) overhead
• No need for SATA interposer cards (for high availability of SATA drives SATA interposer cards are needed)
• Larger (deeper) command queue [depth]

See also[edit]

• List of device bandwidths
• USB Attached SCSI

Computer bus — technical and de facto standards (wired)
General	System bus Front-side bus Back-side bus Daisy chain Control bus Address bus Bus contention Network on a chip Plug and play List of bus bandwidths
Standards	S-100 bus Unibus VAXBI MBus STD Bus SMBus Q-Bus ISA STEbus 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 Extended (PCI-X) AGP PCI Express (PCIe) RapidIO Intel QuickPath Interconnect HyperTransport
Portable	PC Card ExpressCard
Embedded	Multidrop bus AMBA Wishbone
Storage	ST-506 ESDI SMD Parallel ATA (PATA) DMA SSA DSSI HIPPI USB MSC IEEE 1394 interface (FireWire) Serial ATA (SATA) eSATA eSATAp SCSI Parallel SCSI SAS
Peripheral	Apple Desktop Bus HIL MIDI Multibus RS-232 RS-422 RS-423 RS-485 DMX512-A IEEE-488 (GPIB) IEEE-1284 (parallel port) UNI/O ACCESS.bus 1-Wire D²B I²C SPI Parallel SCSI Profibus USB IEEE 1394 (FireWire) Camera Link External PCIe Thunderbolt Lightning
Note: interfaces are listed in speed ascending order (roughly), the interface at the end of each section should be the fastest  Category

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

• Intel
• Micron
• Samsung
• SK hynix
• 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
• USB-IF
• SATA-IO
• SFF Committee
• SNIA
• SSSI
• T10/SCSI
• T11/FC
• T13/ATA

 Category

http://baike.baidu.com/subview/325942/5754229.htm

 

SAS（Serial Attached SCSI），串行连接SCSI接口，串行连接小型计算机系统接口。

什么是SAS ？

SAS是新一代的SCSI技术，和现在流行的Serial ATA(SATA)硬盘相同，都是采用串行技术以获得更高的传输速度，并通过缩短连结线改善内部空间等。SAS是并行SCSI接口之后开发出的全新接口。此接口的设计是为了改善存储系统的效能、可用性和扩充性，提供与串行ATA (Serial ATA，缩写为SATA)硬盘的兼容性。

SAS的接口技术可以向下兼容SATA。SAS系统的背板(Backpanel)既可以连接具有双端口、高性能的SAS驱动器，也可以连接高容量、低成本的SATA驱动器。因为SAS驱动器的端口与SATA驱动器的端口形状看上去类似，所以SAS驱动器和SATA驱动器可以同时存在于一个存储系统之中。但需要注意的是，SATA系统并不兼容SAS，所以SAS驱动器不能连接到SATA背板上。由于SAS系统的兼容性，IT人员能够运用不同接口的硬盘来满足各类应用在容量上或效能上的需求，因此在扩充存储系统时拥有更多的弹性，让存储设备发挥最大的投资效益。

SAS技术还有简化内部连接设计的优势，存储设备厂商目前投入相当多的成本以支持包括光纤通道阵列、SATA阵列等不同的存储设备，而SAS连接技术将可以通过共用组件降低设计成本。

SAS的特点

串行SCSI是点到点的结构，可以建立磁盘到控制器的直接连接。具有以下特点：

1、更好的性能：

点到点的技术减少了地址冲突以及菊花链连结的减速；

为每个设备提供了专用的信号通路来保证最大的带宽；

全双工方式下的数据操作保证最有效的数据吞吐量；

2、简便的线缆连结：

更细的电缆搭配更小的连接器；

3、更好的扩展性：

可以同时连结更多的磁盘设备。

由于串行SCSI（SAS）是点到点的结构，因此除了提高性能之外，每个设备连接到指定的数据通路上提高了带宽。SAS的电缆结构节省了空间，从而提高了使用SAS硬盘服务器的散热、通风能力。一般情况下，较大的并行电缆会带来电子干扰，SAS的电缆结构可以解决这个问题。此外SAS结构有非常好的扩展能力，最多可以连接16384个磁盘设备。

串行SCSI（SAS）硬盘使用与S-ATA相同的接口，但是使用较多的信号，因此SAS硬盘不能与S-ATA硬盘控制器连结。SAS是通用接口，支持SAS和S-ATA，SAS控制器可以支持SAS和SATA磁盘。S-ATA使用SAS控制器的信号子集，因此SAS控制器支持S-ATA硬盘。

初期的SAS硬盘使用2.5英寸封装，这样可以使机架服务器支持更多的硬盘，现在已经有厂商推出标准3.5英寸的SAS硬盘；初期产品的转速是10000RPM，而现在15000RPM的产品也已经问世。SAS硬盘与相同转速的SCSI硬盘相比有相同或者更好的性能。串行接口减少了线缆的尺寸，允许更快的传输速度，SAS硬盘传输数据可以达到3.0Gbit/sec。

每个SAS电缆有4根电缆，2根输入2根输出。SAS可以同时进行数据的读写，全双工的数据操作提高数据的吞吐效率。

SAS的发展史

2001年11月26日，Compaq、IBM、LSI逻辑、Maxtor和Seagate联合宣布成立SAS工作组。

在2003年的CEBIT大会上，惠普和希捷早已推出了SAS界面的硬盘样品。当时，英特尔和Emulex也表示，将计划开发支持SAS和SATA界面的处理器。去年11月，Adaptec也推出了SAS控制器出样，新品的平均数据带宽为3Gbps，峰值带宽达5Gbps。

未来，第二代和第三代的SAS界面将提供6－12Gbps的数据带宽，并支持HostRAID。

现在开发SAS架构的存储设备企业包括希捷、前迈拓、LSI Logic和Adaptec等。

SAS产品市场的发展趋势

在新一代以SAS为基础的应用结构下，SAS与SATA企业用硬盘是彼此能够截长补短非常理想的储存组件。SAS硬盘是为需求量较大及具备关键性处理任务的应用装置所设计的产品，而SATA硬盘则适合于近线储存及其它对于储存需求量较小的中小型企业所应用。

预计今年，低端的存储系统将由SATA取代SCSI硬盘，而高、中端的外部存储系统将大部分采用光纤通道。但存储系统价格的迅速下滑等因素却让业界对SAS硬盘的态度大幅改变。在产品价格快速下降的趋势下，存储设备厂商势必通过更具有成本优势的技术制造存储设备，而SAS硬盘正是符合这种需求的产品。另外，SAS系统和SATA系统的兼容性，以及I-SCSI连接标准的实行，也都会推动SAS系统的发展。

由于企业市场一向对新技术较为保守，也许SAS技术的普及不会像SATA技术那样迅速，但是这也只是时间问题。前迈拓公司预计，到2009年将有三分之二的外部存储设备采用SAS技术，以连接SAS或SATA硬盘。

SAS硬盘应用

存储设备的反应速度，除了各环节间的配合与操作系统的影响之外，硬盘的反应速度其实具有关键性的地位。企业级的工作站或存储设备，一般来说，都采用光纤信道（Fibre Channel，FC）与SCSI硬盘作为内部的存储媒体。但是随着SCSI硬盘在扩增性上的限制，SAS（Serial Attached SCSI）硬盘崭露头角。由于服务器厂商有越来越多采用SAS硬盘作为内部的存储媒体，那么在存储市场里，SAS硬盘是否会成为FC硬盘的劲敌？NetApp表示小型负载的应用可以采用SAS硬盘，可兼具预算与效能的考虑。

既然SAS硬盘比较适合小型负载的应用，那么哪些应用为小型负载的状况呢？NetApp解释，例如在1,000人以下的电子邮件系统，或者规模不大的ERP、CRM系统，很多国内中小企业就相当适合。而像是大型的ERP、CRM系统，或是在线实时交易系统等，因为传输量大，反应速度需要实时快速，所以还是应当采用更高端的光纤信道硬盘。

串行连接SCSI (Serial Attached SCSI，缩写为SAS) SAS是新一代的SCSI技术，和现在流行的Serial ATA(SATA)硬盘相同，都是采用串行技术以获得更高的传输速度，并通过缩短连结线改善内部空间等。 SAS是并行SCSI接口之后开发出的全新接口。此接口的设计是为了改善存储系统的效能、可用性和扩充性，提供与串行ATA (Serial ATA，缩写为SATA)硬盘的兼容性。 SAS的接口技术可以向下兼容SATA。SAS系统的背板(Backplane)既可以连接具有双端口、高性能的SAS驱动器，也可以连接高容量、低成本的SATA驱动器。因为SAS驱动器的端口与SATA驱动器的端口形状看上去类似，所以SAS驱动器和SATA驱动器可以同时存在于一个存储系统之中。但需要注意的是，SATA系统并不兼容SAS，所以SAS驱动器不能连接到SATA背板上。 由于SAS系统的兼容性，IT人员能够运用不同接口的硬盘来满足各类应用在容量上或效能上的需求，因此在扩充存储系统时拥有更多的弹性，让存储设备发挥最大的投资效益。

SAS的接口技术可以向下兼容SATA。具体来说，二者的兼容性主要体现在物理层和协议层的兼容。在物理层，SAS接口和SATA接口完全兼容，SATA 硬盘可以直接使用在SAS的环境中，从接口标准上而言，SATA是SAS的一个子标准，因此SAS控制器可以直接操控SATA硬盘，但是SAS却不能直接使用在SATA的环境中，因为SATA控制器并不能对SAS硬盘进行控制；在协议层，SAS由3种类型协议组成，根据连接的不同设备使用相应的协议进行数据传输。其中串行SCSI协议(SSP)用于传输SCSI命令；SCSI管理协议(SMP)用于对连接设备的维护和管理；SATA通道协议(STP)用于 SAS和SATA之间数据的传输。因此在这3种协议的配合下，SAS可以和SATA以及部分SCSI设备无缝结合。

SAS系统的背板(Backplane)既可以连接具有双端口、高性能的SAS驱动器，也可以连接高容量、低成本的SATA驱动器。所以SAS驱动器和 SATA驱动器可以同时存在于一个存储系统之中。但需要注意的是，SATA系统并不兼容SAS，所以SAS驱动器不能连接到SATA背板上。由于SAS系统的兼容性，使用户能够运用不同接口的硬盘来满足各类应用在容量上或效能上的需求，因此在扩充存储系统时拥有更多的弹性，让存储设备发挥最大的投资效益。

在系统中，每一个SAS端口可以最多可以连接16256个外部设备，并且SAS采取直接的点到点的串行传输方式，传输的速率高达3Gbps，估计以后会有 6Gbps乃至12Gbps的高速接口出现。SAS的接口也做了较大的改进，它同时提供了3.5英寸和2.5英寸的接口，因此能够适合不同服务器环境的需求。SAS依靠SAS扩展器来连接更多的设备，目前的扩展器以12端口居多，不过根据板卡厂商产品研发计划显示，未来会有28、36端口的扩展器引入，来连接SAS设备、主机设备或者其他的SAS扩展器。

和传统并行SCSI接口比较起来，SAS不仅在接口速度上得到显著提升(现在主流Ultra 320 SCSI速度为320MB/sec，而SAS才刚起步速度就达到300MB/sec，未来会达到600MB/sec甚至更多)，而且由于采用了串行线缆，不仅可以实现更长的连接距离，还能够提高抗干扰能力，并且这种细细的线缆还可以显著改善机箱内部的散热情况。

SAS目前的不足主要有以下方面：

 

1)硬盘、控制芯片种类少：只有希捷、迈拓以及富士通等为数不多的硬盘厂商推出了SAS接口硬盘，品种太少，其他厂商的SAS硬盘多数处在产品内部测试阶段。此外周边的SAS控制器芯片或者一些SAS转接卡的种类更是不多，多数集中在LSI以及Adaptec公司手中。

2)硬盘价格太贵：比起同容量的Ultra 320 SCSI硬盘，SAS硬盘要贵了一倍还多。一直居高不下的价格直接影响了用户的采购数量和渠道的消化数量，而无法形成大批量生产的SAS 硬盘，其成本的压力又会反过来促使价格无法下降。如果用户想要做个简单的RAID级别，那么不仅需要购买多块SAS硬盘，还要购买昂贵的RAID卡，价格基本上和硬盘相当。

3)实际传输速度变化不大：SAS硬盘的接口速度并不代表数据传输速度，受到硬盘机械结构限制，现在SAS硬盘的机械结构和SCSI硬盘几乎一样。目前数据传输的瓶颈集中在由硬盘内部机械机构和硬盘存储技术、磁盘转速所决定的硬盘内部数据传输速度，也就是80MBsec左右，SAS硬盘的性能提升不明显。

4)用户追求成熟、稳定的产品：从现在已经推出的产品来看，SAS硬盘更多的被应用在高端4路服务器上，而4路以上服务器用户并非一味追求高速度的硬盘接口技术，最吸引他们的应该是成熟、稳定的硬件产品，虽然SAS接口服务器和SCSI接口产品在速度、稳定性上差不多，但目前的技术和产品都还不够成熟。

不过随着英特尔等主板芯片组制造商、希捷等硬盘制造商以及众多的服务器制造商的大力推动，SAS的相关产品技术会逐步成熟，价格也会逐步滑落，早晚都会成为服务器硬盘的主流接口。

总结

作为一种新的存储接口技术，SAS不仅在功能上可与Fibre Channel媲美，还具有兼容SATA的能力，因而被业界公认为取代并行SCSI的不二之选。据唯实数工程师介绍，SAS的优势主要体现在：灵活性，可以兼容SATA，为用户节省投资；扩展性，一个SAS域最多可以直连16384个设备；性能卓越，点对点的架构使性能随端口数量增加而提高；更合理的电缆设计，在高密度环境中提供更有效的散热。衡量一种技术的优劣通常有4个基本指标，即性能、可靠性、可扩展性和成本。回顾串行磁盘技术的发展历史，从光纤通道，到SATA，再到SAS，几种技术各有所长。光纤通道最早出现的串行化存储技术，可以满足高性能、高可靠和高扩展性的存储需要，但是价格居高不下；SATA硬盘成本倒是降下来了，但主要是用于近线存储和非关键性应用，毕竟在性能等方面差强人意；SAS应该算是个全才，可以支持SAS和SATA磁盘，很方便地满足不同性价比的存储需求，是具有高性能、高可靠和高扩展性的解决方案。