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PMC-Sierra Serial Attached SCSI key in scalable storage

Posted: 14 Oct 2004 ?? ?Print Version ?Bookmark and Share

Keywords:serial attached scsi? sas? sata disk drives? expanders?

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) is to overcome the scalability limitations of parallel SCSI, and improve the ability to add hard disk drives to a system. To overcome the scalability limitation, SAS uses a device called an expander. As described in the following paragraphs, SAS expanders provide scalability, enable fault tolerant systems, provide high throughput and facilitate the compatibility between SAS hosts and SATA disk drives.

Parallel SCSI is a parallel bus architecture and shares connection paths between devices. The shared connection limits the scalability and adding more buses with multiple initiators does little to extend the scalability. Serial Attached SCSI (SAS) is a point-to-point architecture providing ease of scalability. SAS uses expander devices, which are inexpensive switches, to enable quick aggregation of up to 16,384 drives. Expanders simplify the configuration of large external storage arrays and can be cascaded with minimal latency while preserving bandwidth for increased workloads.

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

Serial Attached SCSI expanders maximize performance in the enterprise environment. First generation SAS will deliver throughput of 3Gbps per link and succeeding generations up to 12Gbps 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 Serial Attached SCSI initiators and expanders to increase total available bandwidth. Grouping four or eight links can produce bandwidth of 12Gbps or 24Gbps, respectively.

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

Serial 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 on a physical link.

Currently, the majority of data centers use parallel SCSI storage technology. However, as requirements for higher throughput, increased scalability and improved reliability expand, system integrators will look towards new serial technologies such as SAS. The 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.

- Rachelle Trent

PMC-Sierra Inc.

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