serial attached SCSI

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serial attached SCSI

A standard hardware interface for storage drives. Introduced in 2003, serial attached SCSI (SAS) superseded the parallel SCSI interface and is widely used in datacenters where large numbers of drives are required (see illustration below).

SAS drives are generally faster than SATA drives and more reliable, but are also more costly. Although SATA drives can be connected to a SAS interface, SAS drives require an adapter to hook up to a SATA controller (see SATA and SATA Express).

SAS is a point-to-point architecture that uses a controller (host bus adapter) with two full-duplex channels that operate simultaneously. Each channel (SAS port) transfers data at 3, 6 or 12 Gbps in each direction. In contrast, the consumer-oriented SATA interface has only one channel.

SAS Protocols: SSP, STP and SMP
The Serial SCSI Protocol (SSP) is used for SAS drives, and the Serial ATA Tunneling Protocol (STP) is employed for SATA drives. Expanders use the Serial Management Protocol (SMP). See SCSI and hard disk interfaces.


Massive Scalability
SAS channels (ports) can connect to a single drive or to an expander that connects up to 128 drives (or host bus adapters). A Fanout Expander can connect Edge Expanders, and one system can support 16,256 drives.







Dual Ports for Fault Tolerance
SAS drives use the same form factor as Serial ATA (SATA) for the cable connector but add a second port for connection to two controllers or expanders for fault tolerance.
References in periodicals archive ?
Using JMR's patented PCI-Express based storage technology for higher RAID performance and easy capacity scaling, each DigiLab contains dual RAID controllers and dual SAS expanders that provide higher real world throughput rates than any competing solution.
SAS improves drive addressability and connectivity by using an expander to connect one or more SAS host controllers to up to 128 ports, which may include other host connections, other SAS expanders for even greater scalability, or hard disk drives.
In addition to these features, the SS1300 has been tested for interoperability with the SAS expanders being offered by multiple vendors including Adaptec, LSI Logic, PMC-Sierra and Vitesse.
PMC-Sierra unveiled its max-SAS product family with the latest additions including the highest density 36 and 24 port SAS expanders, the PM8387 SXP 36x3G and PM8388 SXP 24x3G, and an innovative storage enclosure management processor, the PM8389 SMC-L.
Supporting SATA drives through SAS expanders also provides a means of supporting the enterprise necessities of failover and multi-initiator when implementing SATA-based storage systems.
Our SAS expanders leverage PMC-Sierra's industry proven multi-gigabit serial and switching expertise to deliver excellent signal integrity, high performance and low system cost," said Mark Stibitz, vice president and general manager of PMC-Sierra's Enterprise Storage Division.
As more SATA drives and SAS RAID backplanes are added to the enclosure, additional SAS expanders (incorporating table routing) on those backplanes can be cascaded together.
Each expander allows connectivity to 128 physical links, which may include other host connections, other SAS expanders or hard disks.