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.
The SAS expander, in combination with dual-port SAS drives and SATA drives with dual-port failover devices, provides any easy path for system integrators to design redundant systems for high fault tolerance. A complete SAS/SATA storage system solution can be realized by using SAS expander switches combined with the intelligent active/active SATA multiplexers, as shown below in Figure 1. Additional features such a zoning and multi-affiliations are available to system integrators to provide additional flexibility to SAS storage systems.
Multiplexers for redundant applications, fault isolation and diagnostics
SATA active/active multiplexers enable SATA HDDs to appear to be dual port in a SAS system for redundant applications. Furthermore, active/active mux devices provide advanced system-level features such as fault isolation and diagnostics that allow quick and accurate isolation of potential faults. These mux features significantly reduce service times and unnecessary drive returns which result in substantial cost savings for storage system providers and information technology (IT) managers.
Figure 1: SAS/SATA storage system using SAS expander switches with intelligent active/active SATA multiplexers
Industry-Wide Interoperability = System Robustness
Since 2004, six SAS plugfests have been held at the University of New Hampshire’s InterOperability Lab. Servers and storage enclosures as well as discrete SAS components including HBAs, expanders, active/active muxes and HDDs have undergone industry-wide interoperability testing, enabling rapid deployment of robust SAS systems. SAS plugfests are planned to continue into 2007 to ensure testing of SAS-2 requirements including zoning and 6Gb/s data rates.
Proven SAS Expanders and Muxes
SAS expanders with anywhere from six ports up to 36 ports and feature sets targeted towards server and enclosure applications were introduced in 2004, with active/active muxes following close behind. Fully tested for interoperability at a device level at SAS plugfests, expanders and muxes are now being deployed in enterprise server and storage systems.
System Integrators designing SAS-based storage systems have integrated expanders and active/active muxes into their designs in order to attain the scalability, redundancy, capacity and performance that today’s enterprise storage users require. Now is the time to take advantage of the features that expanders and muxes provide and the ease with which they can be designed into a storage system. SAS storage system building blocks are ready for prime time.
Summary
SAS building blocks such as expanders, muxes, HBAs and HDDs entered the market in mid-2004. These devices have been honed through industry wide interoperability testing and system integrator system qualification. As a result, robust SAS server and external storage enclosures are now readily available on the market. SAS is here, it’s robust and IT managers are reaping the benefits.
