System Integrators Can Benefit From a Crash Course in SAS Expanders

Author: Rachelle Trent

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.


sys9fig1.gifFigure 1: SAS Expanders Perform Data Switching Functions Between Multiple Hosts and Target Devices.


Expanders Enable Scalability
Expanders provide the capability to add up to 16, 256 devices to a storage system, overcoming the 15-device limitation of parallel SCSI. In addition, SAS expanders enable fault tolerant systems, provide high throughput and facilitate the compatibility between SAS hosts and Serial ATA (SATA) disk drives. Creating scalable storage systems is now a reality for system integrators with the introduction of several expander devices to the market in 2004, such as PMC-Sierra’s expander product family.

How to Select an Expander
System Integrators (SIs) researching expanders for their storage system designs will find that there are several different features offered by expanders. These features include the number of SAS ports, table routing, latency and integrated enclosure management processors. In general, an expander’s size, throughput and feature set are dictated by the application in which the expander is used, for example a SAS server or an external SAS enclosure.

In an external SAS enclosure, SIs should plan for the enclosure to support a large number of drives, and be capable of cascading enclosures for storage expansion. Some OEMs are planning to cascade up to six external storage enclosures without any negative impact on data throughput. To enable this type of configuration, an expander must provide a large number of routing table entries and provide a very low latency through each expander.

In addition to these requirements, the majority of SIs are looking for an expander that also incorporates an enclosure management processor (EMP). This EMP can manage all LEDs, fans, temperature sensors, etc. and communicate status of the enclosure to the host via SCSI Enclosure Services (SES)-2 commands.

Examples of expanders that are well suited for external storage designs include PMC-Sierra’s 24-port and 36-port SAS expanders, which provide 1024 routing table entries, extremely low latency and an integrated enclosure management processor. Figure 2 depicts how an expander with integrated enclosure management can be used in a 28-drive SAS or SATA HDD enclosure.


sys9fig2.gifFigure 2: Example of a 28-Drive SAS/SATA HDD Enclosure Using 36-port Expanders.


The majority of SAS server applications require low-port-count expanders because the number of HDDs to be supported is low, as shown in Figure 3 below. In a server, the direct-attached storage is not cascaded, removing the need for the expander to support a large routing table or provide extremely low latency. In addition, server designs have simpler enclosure management requirements, as they typically need only LED lights. For this application, low-port-count, low-cost expander solutions are ideal. PMC-Sierra’s product line includes 6-port and 12-port SAS expanders suitable for server applications.


sys9fig3.gifFigure 3: Example of a Sever Application Using SAS Expanders.


Expander Interoperability Testing Successful
SIs planning to use expanders in their system designs are concerned with interoperability with HBAs, HDDs and other expander products. During 2004, three SAS plugfests were held at the University of New Hampshire’s InterOperability Lab. HBA, expander, HDD, cable, connector and test equipment vendors participated in all three events, resulting in industry-wide interoperability and the successful adoption of SAS in the storage market. In 2005, three more SAS plugfests are planned to continue interoperability testing, focusing on system-level interoperability.

Expanders are Ready for Prime Time
SAS expanders with 6 ports up to 36 ports and feature sets targeted towards server and enclosure applications were introduced in 2004. Fully tested for interoperability at a device level in 2004, expanders are now ready for use in enterprise storage systems.

System Integrators planning to design SAS-based storage systems will need to integrate expanders into their designs in order to attain the scalability, capacity and performance that today’s enterprise storage users require. Now is the time to understand the advantages that expanders provide and the ease with which they can be designed into a storage system, as SAS storage will be here soon.

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