STA

Author: Rachelle Trent, SAS Product Manager,
PMC-Sierra

As enterprise storage requirements rapidly increase, expanding data throughput and providing scalable storage capacity is crucial. The goal of Serial Attached SCSI (SAS) was to overcome the scalability and reliability limitations of parallel SCSI and to significantly improve the ability to add hard disk drives to a system. To overcome the scalability limitation, SAS uses devices such as expanders and multiplexers (muxes).

Expanders for scalability
SAS expanders provide scalability and enable fault tolerant systems by providing multiple connections between hard disk drives and initiator devices. These redundant links between hosts and drives maintain a connection in case of a system failure. In addition, SAS expanders provide high throughput and facilitate the compatibility between SAS hosts and Serial ATA (SATA) disk drives. Expanders simplify the configuration of large external storage arrays and can be cascaded with minimal latency while preserving bandwidth for increased workloads.

Author: Sam Barnett, Product Line Manager, Serial Attached SCSI
and Serial ATA Storage & Storage Networking Products, Storage Products Division,
Vitesse Semiconductor

Serial Attached SCSI (SAS) offers a wealth of benefits to the enterprise server and enclosure customer (high reliability and performance, mixed enterprise/desktop drive support, improved economies of scale), and key to scalability and performance is the expander.

This article offers a brief technical exploration of SAS expander technology and its potential evolution into the "interconnect-fabric" of choice for midrange and enterprise storage systems of tomorrow.

The author assumes the reader has some knowledge of point-to-point storage architectures and SAS nomenclature. Detailed specification information on SAS can be found on the T10 website under http://www.t10.org/ (click "Drafts" then "Serial Attached SCSI (SAS)" under "Protocols and Physical Layers"). Similarly, the SCSI Trade Association (STA) also offers a robust set of tutorials on SAS. The reader may reference this material at http://www.scsita.org.

Author: Sam Barnett, Product Line Manager for Serial Attached SCSI and Serial ATA,
Vitesse Semiconductor

The NexSAS™ family of Serial Attached SCSI (SAS) products from Vitesse offers server and storage enclosure OEMs, ODMs, and contract manufacturers (CMs) the industry's most comprehensive portfolio of solutions for the emerging SAS market. Comprised of edge expanders, fanout expanders, RAID-on-CHIP offerings, intelligent mux/de-mux devices, discrete enclosure management/SAS backplane controllers, and rate-agile signal conditioners, the NexSAS™ line delivers on the promise of high performance storage solutions for SMB (small/medium business) as well as enterprise systems.

The Vitesse NexSAS™ product family is divided into four functional areas: expander technology, RAID-on-CHIP, SAS/SATA Support devices, and Enclosure Management. This article explores the NexSAS™ expander and RAID-on-CHIP technology in detail.

Rachelle Trent, SAS Product Marketing Manager, PMC-Sierra

As enterprise storage requirements rapidly increase, expanding data throughput and providing scalable storage capacity is crucial. System Integrators (SIs) need to understand the new features of expander devices now, in order to be ready for market adoption of Serial Attached SCSI (SAS). The SAS standard specifies expanders, which are inexpensive switch devices connecting SAS hosts to SAS targets as shown in Figure 1 below.

Figure 1: A SAS Expander performs a data switching function between multiple hosts and target devices.

By Rachelle Trent
PMC-Sierra

As enterprise storage requirements rapidly increase, expanding data throughput and providing scalable storage capacity is crucial. One of the goals of Serial Attached SCSI (SAS) was to overcome the scalability limitations of parallel SCSI and significantly improve the ability to add hard disk drives to a system. Parallel SCSI is a shared bus architecture that utilizes common connection paths between devices. The shared connection limits scalability, and adding more buses with multiple initiators does little to extend the scalability.

Author: Rachelle Trent
PMC-Sierra

As enterprise storage requirements rapidly increase, expanding data throughput and providing scalable storage capacity is crucial. System Integrators (SIs) need to understand the advantages and architecture of devices called "expanders" now, in order to be ready for market adoption of Serial Attached SCSI (SAS). The SAS standard specifies expanders, which are inexpensive switch devices connecting SAS hosts to SAS targets as shown in Figure 1 below.

Figure 1: SAS Expanders Perform Data Switching Functions Between Multiple Hosts and Target Devices.

Author: Mike Micheletti
Computer Access Technology Corp. (CATC)

Scalability can be defined as "the ease with which a solution or component can be modified to fit the problem as the size of the problem increases". Scalability in the storage domain allows IT managers to respond to changing storage requirements within the enterprise, and is a stated design goal of Serial Attached SCSI (SAS).

The SAS protocol delivers scalability via Expanders, integrated circuits (IC) that function as switch components and route frames between initiators and end-points. By providing multiple point-to-point drive connections in a single IC, expanders allow aggregation of bandwidth between Initiator and multiple SAS disk drives. Expanders also enable an architecture that accommodates dynamic discovery and configuration as new components are added to the system.

Author: Paul Griffith
Adaptec

SCSI is the most versatile disk interface used today for mass storage in enterprise environments by virtue of the protocol's advanced features, throughput capabilities, and support for large storage infrastructures. But the growing need for faster, more complex data communications and more scalable storage systems raises the question of whether parallel SCSI can support the increasing capacity scalability requirements needed for external storage systems.

The physical layer of the parallel SCSI bus has continued to limit its ability to widely scale. While standard unassisted Low Voltage Differential (LVD) SCSI supports a maximum of 12 meters between the computer and external device, many of today's larger SCSI systems require cables longer than 12 meters; hence, the need for SCSI bus expanders.