The Future of SAS-based Storage Systems

Author: Tonya Comer, HP

Three years ago the Serial Attached SCSI (SAS) and Serial ATA (SATA) industry working groups developed specifications for serial I/O technologies that will provide the flexibility and performance that IT managers need for storage products and solutions in the future. SCSI Trade Association (STA) member companies have worked together in the interim developing SAS technology to ensure quality products for reliable, quick-time-to-market SAS solutions. SAS solutions will be implemented in enterprise-class data centers, delivering the full functionality of enterprise-class systems and subsystems such as hard disk drives (HDDs). SATA solutions will be implemented in desktop PCs and low-cost, non-mission-critical server storage solutions.

SAS Products Are Entering the Market
SAS system controller products will be deployed as either internal or external device links. In addition, there will be a broad range of controller products available to the market, ranging from embedded solutions to high-performance cards. All SAS devices can have one or more ports, with each port configured as a narrow (single link) or wide (multiple links) port. The SAS architecture supports up to 16,000 links within a single domain, each link with a data transfer rate of up to 300 MB/s.

The SAS interface allows devices to have two full duplex links, which are limited to no more than two simultaneous data transfers. Second-generation SAS devices will have two ports to support redundant paths for high-availability configurations. Expander products will allow controllers to connect to a greater number of devices by providing up to 128 physical links (Figure 1). These links may include SAS and/or SATA devices, other expanders and other host connections.

Figure 1: Serial Attached SCSI Components


SAS Architecture Supports SAS and SATA Devices
The SAS architecture enables system and product designs that deploy both SAS and SATA devices, a breakthrough for enterprise customers. This provides a broad range of storage solutions that give IT managers the flexibility to choose storage products based on reliability, performance, and cost. SAS and SATA devices will share the same physical device connector, except for an extension on the SAS connector. This extension will allow SAS to accept SATA device connections. However, SATA will not accept SAS device connections based on the current specification (Figure 2). High-performance and highly reliable SAS disk drives can be used for mission-critical and performance-oriented applications, while high-capacity SATA drives can be used for disk-enhanced backup or non-mission-critical reference data.

Figure 2: SAS System Backplane Flexibility


SAS enables a wide range of products and highly scalable solutions—internal, external, or a combination of both—to give manufacturers and customers the flexibility to design and deploy a range of solutions.

Figure 3 shows an internal RAID solution, similar to what SAS quick-time-to-market server products will deploy. The SAS RAID controller can support two or more internal HDDs, which can be either SAS or SATA drives. The small SAS connector will allow connections to 3.5-inch or 2.5-inch internal HDDs, enabling redundant storage configurations in dense server form factors like 1U, 2U, and blade servers.

Figure 3: Topology for Internal Storage Applications


The SAS interface broadens choices for customers by enabling the ability to plug either SAS or SATA drives into one SAS JBOD backplane. This will give customers the flexibility to configure drive arrays with SAS or SATA, or both, enabling the use of enterprise drives and desktop drives in the same server or shared storage subsystem (Figure 4).

Figure 4: Topology for Internal and External Storage Systems


The benefits of SATA and SAS will extend from desktop products in the data center (including investment protection in SCSI software), to higher HDD performance, smaller form factors, and greater device addressability. Because SATA and SAS architectures use the same physical device connector, customers have the flexibility to design solutions that use both SAS and SATA devices. This flexibility is crucial for the adaptive enterprise.

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