SCSI (Small Computer System Interface), often called Parallel SCSI, is almost 30 years old and can hardly keep up with the demands of today’s IT environment. Serial Attached SCSI (SAS) was developed to address the limitations inherent in SCSI. This article highlights the differences between these two interfaces and points out the attributes that account for the increasing popularity of SAS.

Shiv Dutta, Senior Software Engineer, IBM, Software Group

Shiv Dutta is a Solutions Relationship Manager in the IBM Systems and Technology Group where he is responsible for the ISV relationships for all IBM Tape Storage Products. Shiv has worked extensively in the System p and AIX area.He is a co-author of a couple of IBM Redbooks on AIX. You can reach him at sdutta@us.ibm.com.



16 December 2008

Also available in Chinese

Introduction

SCSI (Small Computer System Interface) is a set of standards for physically connecting computers and peripheral devices and transferring data between them. These standards define commands, protocols, electrical, and optical interfaces. SCSI is most commonly used for hard disks and tape drives, but it can be used for a range of other devices, such as scanners, CDs, and DVDs.

SCSI, often called parallel SCSI, is based on bus technology. It is almost 30 years old and can hardly keep up with the demands of today’s IT environment. For example, it maxes out at 320 MB/sec (Ultra320 SCSI) and its performance suffers as more and more devices are added to the shared bus, neither of which can be compromised due to the computational complexities in the prevailing corporate IT requirements.

Serial Attached SCSI (SAS) was developed to address the I/O and direct-attach storage requirements that the traditional parallel SCSI could not meet. On the one hand, it offers logical compatibility with SCSI. On the other, it provides the reliability, performance, and manageability that practitioners have come to expect of SCSI. Like parallel SCSI, it is a data-transfer technology designed to move data to and from computer storage devices such as hard drives and tape drives. Unlike SCSI, which is multi-drop, SAS is a point-to-point protocol and allows for much higher speed data transfers than has been possible with parallel SCSI. It uses the standard SCSI command set for interacting with SAS End devices.

The SAS protocol was developed and is maintained by the T10 technical committee of the International Committee for Information Technology Standards (INCITS).


Components

A typical SAS system consists of the following four basic components:

  • Initiator
  • Target
  • Service Delivery Subsystem
  • Expanders

An Initiator is a device that sends device service and task management requests to a target device and receives responses for the same requests from the target device. Initiators may be provided as an on-board component on the motherboard or as an add-on host bus adapter.

A Target is a device that contains the logical units and target ports. It receives device service and task management requests for processing and sends responses back to the initiator that transmitted these requests. A target device could be a hard disk or a disk array system.

A Service Delivery Subsystem (SDS) is part of the I/O system. It transmits the information going back and forth between an initiator and the target. Generally, an SDS consists of the cables that connect an initiator to the target with or without expanders.

Expanders are devices that are part of an SDS. They facilitate communication between the SAS devices. They also facilitate connection of multiple SAS devices to a single initiator port.


Parallel SCSI versus SAS

The following table illustrates the main differences in properties between SAS and SCSI interfaces:

Parallel SCSI versus SAS

Parallel SCSISAS
Architecture Parallel, all devices connected to shared bus. Serial, point-to-point, discrete signal paths. Port expander1 used for fan-out.
Performance Max speed 320 MB/sec (Ultra320 SCSI). Performance degrades as devices added to shared bus. Speed shared across the entire multi-drop bus. 3.0 GB/sec, roadmap to 12.0 GB/sec. Performance is maintained as more drives are added.
Scalability Number of devices per cable limited by SCSI IDs to 8, 16, or 32 on a single channel. Up to 128 devices. 16,384 devices with fan-out expander.
Compatibility Incompatible with all other drive interfaces. Compatible with Serial ATA (SATA).
Max. Cable Length 12 meters total. Can use SCSI repeaters to exceed this limit but they are expensive. 8 meters per discrete connection; total domain cabling thousands of feet.
Cable Form Factor Multitude of conductors adds bulk an cost. Compact connectors and cabling save space and cost.
Hot Plug ability Not optimized. Some care required. Yes.
Device Identification Manually set; user must ensure no ID number conflicts on bus. Worldwide unique ID set at time of manufacture uniquely identifies devices; no user action required.
Termination Manually set; user must ensure proper installation and functionality of terminators. Discrete signal paths enable devices to include termination by default; no user action required.

1Port expanders are essentially switches with powerful processors in them.


Conclusions

From the end-user point of view, SAS provides enterprise-class robustness, protection of investments in compatible SCSI software and applications, and, because of its compatibility with SATA (Serial Advanced Technology Attachment), SAS offers a choice of direct attaché storage devices in a single SAS system. Additionally, since it is based on a serial interface, SAS allows for increased device support.

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