Originating Author: Gina Geisel
A typical shared or networked storage environment may consist 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 Serial Attached SCSI (SAS), Fibre Channel (FC), and Internet Protocol SCSI (iSCSI) to name a few. 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. 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. With already a large install base of SAS drives in place, the industry has also accepted SAS as the shared Direct Attached Storage (DAS) interconnect of choice. And with the introduction of the SAS switch, organizations can acheive the consolidation benefits as a FC or IP SAN with SAS, at lower acquisition and operational costs.
SAS interface technology offers the following value propositions:
Increased aggregate performance - SAS technology supports faster data throughput with port (sometimes called a link) aggregation using x4-wide ports and full duplexing. Each individual port operates at 6Gb/s which provides a theoretical cumulative bandwidth of up to 48Gb/s when all lanes are shipping frames containing I/O and data. Figure 1 shows that links 2, 3, and 4 are used as overflow when concurrent I/Os overload the previous link. .
The following figure displays a x4-wide port, which is the standard external connection for SAS.
Simple Storage Consolidation - SAS supports a shared direct-attached storage configuration and offers improved efficiency by enabling busy storage administrators to reduce the number of individual systems that need to be managed and maintained. As organizations grow and add additional servers to their infrastucture, a SAS switch can be introduced for SAN-like consolidation capabilities with out the cost of a FC SAN or the performance constraints of an IP SAN.
Improved Reliability, Point-to-Point and Dual Porting - Point-to-point storage architectures, which the SAS interface adopted, proved to be easier to initialize, troubleshoot, and maintain. The dual porting capability of FC had proven to be successful and therefore became the basic failover model adopted by SAS.
Affordable and lower TCO – Priced at price point comparable to SCSI, SAS ensures a low initial investment with the ability to support lower TCO with Reduced administration management and hardware costs with management of a single shared storage array is more efficient and requires less resources then distributed storage
High availability – SAS offering clustering support for external storage with OSs that do not support internal DAS clusters ensuring higher availability than previous SCSI DAS implementations.
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: