Originating Author: Gina Geisel
Interface Choices
A typical shared or networked storage environment consists of application hosts, storage devices, and external hardware interfaces within the application server, the appropriate cabling, and a switch between the hosts and storage systems.
The external interface technologies, as components of these environments, are the foundation of the overall storage framework’s performance, scalability, reliability, technical complexity, and cost. The industry has developed several interface options to support environments such as these, including Serial Attached SCSI (SAS), Fibre Channel (FC) protocol, and Internet Protocol SCSI (iSCSI). Each of these interfaces has their own distinct features and characteristics. This article examines the strengths, positioning, special considerations, and applications of the SAS storage environment.
SAS Interface Overview
With a solid roadmap and industry acceptance behind it, SAS technology is proving to be the leading follow-on of parallel SCSI technology. SAS satisfies business’ interface requirements of scalability, performance, reliability, and manageability at an affordable price-point. The industry has accepted SAS as the Direct Attached Storage (DAS) interconnect of choice. SAS technology provides storage connection for both internal and external configurations. In its simplest internal configuration, SAS provides a physical connection between a host initiator and a target device such as a hard disk drive. In an external system configuration, SAS provides a physical connection between storage systems and multiple hosts.
Providing unprecedented performance and scalability for server direct attached storage and external storage systems environments, SAS controller ICs meet the increasing bandwidth and flexibility demands of blade server environments and high-end server and storage subsystems. SAS controller ICs today support 1.5Gb/s, and 3Gb/s SAS and SATA data transfers per port, SAS and SATA drive connectivity, and wide-port support for aggregated bandwidth across multiple SAS phys. The SAS technology roadmap shows 6Gb/s and even 12Gb/s SAS in the future.
Benefits of SAS
SAS is a serial, point-to-point, enterprise-level device interface that leverages the proven SCSI protocol set. SAS is a convergence of the advantages of SATA, SCSI, and FC, and is the future mainstay of the enterprise and high-end workstation storage markets. SAS offers a higher bandwidth per pin than parallel SCSI and improves signal and data integrity.
The SAS interface uses the proven SCSI command set to ensure reliable data transfers, while providing the connectivity and flexibility of point-to-point serial data transfers. The serial transmission of SCSI commands eliminates design challenges with clock skew and crosstalk. The SAS interface provides improved performance, simplified cabling, smaller connectors, lower pin count, and lower power requirements when compared to parallel SCSI.
SAS controllers leverage a common electrical and physical connection interface that is compatible with Serial ATA technology. The SAS and SATA protocols use a thin, seven wire connector instead of the 68-wire SCSI cable or 26-wire ATA cable. The SAS/SATA connector and cable are easy to manipulate, allow connections to 2.5” small form factor disk drives, and do not inhibit airflow.
SAS Positioning
SAS interface technology offers the following value propositions:
- Affordability – When designing SAS technology, a key element was to keep its cost in-line with its SCSI predecessor. Designers successfully achieved this cost objective, and SAS is an affordable enterprise storage solution for all types and sizes of organizations.
- Flexibility – SAS supports both SAS and SATA disk drives. This allows IT managers to specify the most appropriate drive based on the need: SAS for online transactional, high performance, and high availability; or SATA for nearline, archival, reference, and low availability. Such flexibility minimizes system development and infrastructure redundancy, reducing hardware and IT management costs.
- Performance – SAS technology supports faster data transfer with port aggregation using x4-wide links. Each port supports up to 3Gb/s, which provides a x4-wide link with a theoretical cumulative bandwidth of up to 12Gb/s.
The following figure displays a x4-wide link, which is the standard external connection for SAS.
- Scalability – SAS expander devices are essentially high-speed switches, each one capable of connecting as many as 128 end devices. With SAS expanders, enterprise systems may reach drive counts over 16K.
- Dual port functionality – SAS disk drives are dual-ported, allowing connections to more than one host and eliminating single points of failure.
Action Item: Today, the industry offers a broad range of SAS solutions, ranging from controller and expander ICs, to Host Bus Adapters (HBAs) and RAID adapters, to external storage, to active-active multiplexers, to drive controller solutions. Based on the many advantages of SAS discussed, it is important to determine where each of these solutions will provide the greatest benefit within your infrastructure.
Footnotes: For more information about LSI Serial Attached SCSI products, please visit:
http://www.lsi.com/storage_home/index.html
