The Benefits of Serial Attached SCSI (SAS)



The next generation of SCSI technology, Serial Attached SCSI (SAS), will bring more flexible storage solutions to end users and storage/systems integrators in a variety of ways. Although SAS was not the first attempt to serialize the SCSI protocol, it is the first standard specification to provide an interconnect mechanism for both SCSI and Serial ATA (SATA). As such, SAS meets both enterprise and midrange/nearline storage requirements at relatively low costs, providing users and integrators with flexible storage architectures.

Unlike its parallel SCSI predecessors, the SAS protocol provides a tunneling mechanism for delivering SATA frames through SAS connection infrastructures, including physical cabling connections. These interchangeable drive and cable connectors ensure “plug and play” between both SAS and SATA targets (hard disk drives, DVD drives, tape, etc.). Similarly, protocol and connection compatibility allows OEMs to easily configure storage systems for multiple markets by simply altering disk drive options and characteristics. A single SAS enclosure or server can be configured to support enterprise and/or midrange/nearline applications. In addition, end users can easily configure or update systems by swapping high-performance SAS drives and low-cost SATA drives as appropriate.

Serialization of the SCSI interface overcomes the physical and functional limitations of parallel interface technology. Increased bandwidth requirements, as well as challenges presented by clock skew and power consumption, prevented SCSI from moving beyond the Ultra320 specification. Serialization addresses the parallel interface limitations while significantly reducing power consumption. SAS leverages technologies prevalent in other serial interfaces, including SATA, Gigabit Ethernet, and Fibre Channel.

Initial SAS systems will support a 3Gbps data rate for SAS drives and will be compatible with SATA I (1.5Gbps) data rates through an idle interleave process. Bandwidth scalability is addressed in SAS via connection aggregation. In this scheme, each SAS connection between common endpoints acts as a logically bonded channel, and bandwidth increases as individual channels are combined. These aggregated connections are referred to as “wide ports” with common connection counts of 2 and 4. For example, a “x4” wide port aggregates four independent physical links and provides up to 12MBps of bandwidth.

Similarly, port aggregation provides for physical connection fail-over since each SAS connection acts independently. If a single connection in a wide port is lost, only the bandwidth is compromised, not the entire connection or session service. This feature is unique to SAS and is key to providing scalable storage systems.

The first SAS deployments are targeted at servers with direct-attached storage (DAS). In this model, SAS hard drives and other targets are attached to the host via one of two methods. The simpler method connects drives directly via point-to-point connections to the host controller without expansion devices. In this configuration, the SAS host controller is usually on the motherboard and provides varying levels of RAID for data protection.

In the DAS model, scalability and physical connections are limited to the number of physical interface ports on the embedded SAS controller. Today, controllers typically provide either four or eight connections to hard drives or other targets. Multiple drive types (SAS and SATA) can be intermixed to suit application and performance requirements.

A second model allows for more flexible and scalable designs that take full advantage of system bandwidth. In this architecture, a SAS expander provides modular connectivity (in groups or cascades) for both SAS and/or SATA targets. This “expanded” model provides for significant flexibility in server and storage architectures.

External SAS storage topologies vary with OEM and end-user requirements but one key theme exists: External enclosure designs must be based on architectures that support both SATA and SAS drives. As such, the same enclosure can be targeted at two distinct markets while reducing design skews.

Fail-over is achieved via the connection scheme and the use of dual-port drives. While SAS drives are natively dual ported, SATA drives require a dual-port multiplexer.

External SAS storage systems are typically based on one of two models. One model uses a daisy-chain topology while the other leverages a star or switched approach. The expandable daisy-chain model cascades external SAS or SATA JBODs. This model is the more cost effective; however, latency increases as the number of JBODs or other targets increase. The higher-performance model uses a switched architecture with a “fan-out” expander to provide for a highly scalable architecture with no increase in latency as JBODs are added. Both allow for customized storage systems that can provide enterprise level, high-transaction services with SAS drives or nearline storage with low-cost SATA drives.

Additionally, SAS architectures will address large storage capacities in relatively dense enclosures by taking advantage of the 2.5-inch (SSF) drive form factor. Increasing spindle counts increases performance in striped RAID configurations across either SAS or SATA drives.

SAS addresses bandwidth requirements not only by its serial architecture, but also through aggregation schemes that logically bind multiple connections together. This model provides multiple choices for system OEMs and integrators when tradeoffs among power, pin count, and bandwidth are required.

Features found today in enterprise-level SANs are creeping into SAS components. One example is comprehensive diagnostics. As SAS architectures mature, more complete sets of diagnostics and tools will be required for debugging of subsystems during development and also for installed production equipment.

SAS expanders enable storage network fabrics. SAS architectures support numerous storage targets as well as multiple hosts. In a fabric architecture, access control techniques become critical. SAN features such as zoning are now being addressed by standards organizations. Zoning will be particularly important for disk-less blade environments, where each blade has its own protected boot drive in an external enclosure. Future enhancements to SAS will likely include encryption and virtualization.


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