Author: Tonya Comer, Product Marketing Manager
Industry Standard Servers, HP

Serial Attached SCSI (SAS) is a point-to-point disk interface that builds on the functionality of SCSI for the enterprise. Borrowing reliability and performance enhancements from Fibre Channel, SAS supports dual-port and full-duplex communication between servers and disk drives. SAS extends reliability and bandwidth opportunities by using expanders to create wide ports, which are multi-path connections between the server and storage solutions. SAS also improves manageability of the storage solution by providing the Serial Management Protocol (SMP) to monitor connections between individual devices and identify points of failure or reduced performance.

The first generation of SAS achieves bandwidth as high as 300 MB/s per link and operates in full-duplex mode, in which data, commands, or status information flow bi-directionally. The SAS interface allows for combining multiple links to create 1x, 2x, 3x, or 4x connections for scalable bandwidth. In contrast, Ultra320 SCSI has a half-duplex bandwidth of 320 MB/s per channel.

Figure 1 illustrates the bandwidths of SAS and Ultra320 SCSI compared to the bandwidth requirements of a number of 15K RPM drives. Notice that Ultra320 SCSI (per channel) can handle the sustained transfer rate (STR) of a maximum of four 15K RPM RAID drives while 4x SAS meets the requirements of up to 16 drives.

Figure 1. Bandwidths of SAS links and Ultra320 SCSI compared to the sustained transfer rate of 15K RPM HDDs

SAS drives are available as 3.5-inch hard drives and new 2.5-inch small form factor (SFF) enterprise hard drive technology for space-efficient storage. Historically, the parallel SCSI hard drive (3.5-inch) has been one of the industry-standard storage technologies because of its reliability. Now customers will generally choose SAS hard drive solutions for higher data integrity, performance, reliability, and hot-plug capability. Many customers will opt for SFF SAS technology for low-power and higher performance from more spindles in a given space. Other benefits of SFF SAS enterprise drives include:

• More drives in a server, requiring 70% less space while utilizing more spindles per U to deliver higher performance
• Designed for I/O-intensive environments
• Lower power consumption, uses 30% less energy to ease cooling burden
• Designed with universal SAS hot-plug architecture that rapidly deploys all server and storage platforms, allowing a customer to have only one drive across all segments

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

Internal Topology:
Figure 2 shows a topology that can be used for internal RAID systems. The SAS RAID controller can support from two to eight internal HDDs, which can be either SAS or SATA devices. 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 such as blade servers.

Internal/external with JBOD Topology:
The SAS interface will offer new levels of choice by enabling customers to plug either SAS or SATA drives into one backplane. This will give customers the flexibility to configure drive arrays with SAS, SATA, or both, enabling the use of enterprise devices and desktop devices in the same server or networked storage subsystem as demonstrated in Figure 3.

 
Multi-Node Clusters Topology:
The highly scalable architecture enables topologies that support multi-node clustering for high availability (failover) or load balancing (Figure 4).