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Category: Computers
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Published on Monday, 02 August 2010 19:48
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Written by Administrator
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Serial Attached SCSI (SAS) is a computer bus 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 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.A typical Serial Attached SCSI system consists of the following basic components:
- 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.
- 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.
- 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.
- 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.
SAS vs SATA
- SAS is full-duplex, SATA is half-duplex.
- 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 supports 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. ATA directly supports only 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 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.
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 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 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.
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:
- Supports three transport protocols:
- Serial SCSI Protocol (SSP): supports SAS devices
- Serial ATA Tunneled Protocol (STP): supports SATA devices attached to SAS expanders
- Serial Management Protocol (SMP): provides for the configuration of SAS expanders
- Application layer
