The Importance of Expanders to System Integrators

By Rachelle Trent

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.

Expanders Provide Scalability
SAS is a point-to-point architecture providing ease of scalability through the use of expander devices, inexpensive switches that enable quick aggregation of up to 16,384 drives. SAS expanders not only provide scalability, they also enable fault tolerant systems, provide high throughput and facilitate the compatibility between SAS hosts and Serial ATA (SATA) disk drives. Furthermore, expanders simplify the configuration of large external storage arrays, and can be cascaded with minimal latency while preserving bandwidth for increased workloads.

Expanders Enable Fault Tolerance
SAS expanders can also enable fault-tolerant designs by enabling multiple connections between hard disk drives and initiator devices. These multiple connections provide redundant links between hosts and disk drives to maintain a connection in the event of system failure. The SAS expander, in combination with dual-port SAS drives and SATA drives with dual-ported failover devices, provides an easy path for system integrators to design redundant systems for high fault tolerance.


4_sysint.gifFigure 1: Expanders Increase Scalability and Fault Tolerance
in a SAS System


Expanders Increase Performance
Serial Attached SCSI expanders maximize performance in the enterprise environment. First generation SAS will deliver throughput of 3Gb/sec per link and succeeding generations up to 12 Gb/sec to keep pace with technology and application advances. SAS expanders allow individual links to be combined to create wide ports, allowing system designers to aggregate the performance of SAS initiators and expanders to increase total available bandwidth. Grouping four or eight links can produce bandwidths of 12 Gb/sec or 24 Gb/sec, respectively, greatly increasing system performance.

Expanders and SATA Interoperability
Another important advantage of SAS expanders is interoperability with SATA disk drives. This feature is ushering in a new approach to storage system design, enabling system integrators to populate a single SAS storage enclosure with both SAS and SATA disks. Enterprise-class storage systems can be equipped with high performance SAS drives for mission-critical applications. The same storage enclosure can also be deployed with lower cost SATA drives to address segments of the market that value capacity over performance.

Serial ATA Tunneling Protocol (STP) is the foundation for supporting SATA devices within SAS storage enclosure subsystems. Expanders attach directly to SATA drives and tunnel SATA frames over SAS for transport to the SAS initiator. Once an STP connection is open, the SATA host capability (within the SAS HBA) and the SATA device communicate as if they were directly attached by a physical link.


4_sysint_2.gifFigure 2: Expanders Permit SATA Drives to Communicate Directly
with the Host


Currently, the majority of data centers use parallel SCSI storage technology. However, as requirements for higher throughput, increased scalability and improved reliability grow, system integrators will look towards new serial technologies such as SAS. SAS expanders are a key component of the SAS infrastructure in the mainstream enterprise storage arena. SAS expanders deliver the highest scalability, with support for more than 16,000 devices, and provide storage configuration flexibility by supporting both SAS and SATA disk drives. Expanders also enable system integrators to design fault-tolerant systems with redundant pathways, and provide increased throughput by allowing individual links to be combined to create wide bandwidth pipes.

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