Storage: Sending Out an SAS

High speed, high bandwidth and high availability--all are on the "must-have" list for advanced media storage technologies.
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High speed, high bandwidth and high availability--all are on the "must-have" list for advanced media storage technologies.

Since the early 1990s, disk drive performance, size and applications have continued to develop at a record pace. From early parallel SCSI (Small Computer Systems Interface) to Fibre Channel and beyond, more choices are becoming available to storage architects, including a recent and rather aggressive transition from those root concepts to much faster, larger, less costly and more flexible hard drives.

Fibre Channel, first developed in the late 1990s to meet requirements for high-speed connectivity, has rapidly became the enabling technology for SANs (storage area networks), providing improvements in consolidation, utilization, and manipulation of block storage. Less than a couple decades before, parallel SCSI was the enabling technology. Today, that parallel technology is being replaced by SAS (serial attached SCSI) and its cousin SATA (serial ATA).

Fibre Channel drives now have throughput of 2 Gbps, with 4 Gbps data rates being introduced. Fibre Channel proponents still contend that the technologies will remain more reliable than Ethernet-based solutions, leveraging higher availability and built-in redundancy. For media server applications, Fibre Channel remains the predominant solution for those mission-critical, online/on-demand applications. However, cost remains a negative factor in large Fibre Channel architectures--for drives, enclosures, qualifications and external peripherals such as switches and controllers.

In 2002, SATA provided an alternative, lower cost path for high-volume desktop-class storage applications. Since that time, SATA has continued to make headway into the nearline media storage arena; providing suitable reliability with a primary focus intended for applications that were not mission-critical.

The recent introduction of SAS is the outcome of three more years of advanced study and innovation. SAS is touted as a high-performance solution with greater flexibility at a lower price, and reliability on par with Fibre Channel serial technology. SAS is targeted at enterprise storage applications and sits economically between the higher-end Fibre Channel and the lower-end SATA continuum.


In general, employing serial technology (versus older parallel SCSI approaches) improves data transfer because skew and timing errors no longer inhibit performance. Just as in serial digital video transports (i.e., SMPTE 259M or 292M), bits arrive serially--in order--and at a faster rate. SAS provides a reliable point-to-point connection with a physical link rate of either 1.5 Gbps or 3 Gbps. A roadmap to 12 Gbps is already in place.

The SAS physical link can operate in dual-simplex (full-duplex) mode, with total bandwidth up to 600 MBps (300 MBps in each direction). Serial technology's point-to-point architecture employs a dedicated connection that delivers full bandwidth to each device. It should be noted that the connection rate is always less than or equal to the physical link rate--thus, if a connection is slower than the physical link, rate matching is used. On a per-port basis, the SAS standard allows up to 16,256 addressable devices per port.

SAS employs compact connectors and thinner cabling, which enables higher drive density packing and improved cooling metrics. A maximum connection length of 8 meters per discrete connection is achieved through LVDS (low voltage differential signaling). LVDS further reduces vulnerability to RF interference and achieves speeds unobtainable when using higher voltage signaling. SAS will support SATA drive technologies, providing an avenue to storage in a single common enclosure. The drive upgrade path lets a user start out with the lower cost SATA drives and move up to SAS should performance requirements or other needs change; and allows a step toward Fibre Channel performance without necessarily changing the enclosure.

SAS failover protection is through a direct dual-port connection to the drive. NCQ (native command queuing) supports logic unit numbers greater than 2 TB. In principle, NCQ lets a drive set up a DMA (direct memory access) operation for a non-host intervention data transfer; letting the drive control the DMA engine, which minimizes latencies and optimizes command ordering. Essentially, the drive now controls the order in which reads and writes are executed.

Furthermore, for NCQ to function in some chipsets, it must be enabled at the motherboard level through the advanced host controller interface in the BIOS, requiring the appropriate accelerator software. Note that these technologies are also used in SATA drives.

Additional benefits mean that users no longer need to set drive identification assignments, as there is a worldwide unique ID set at the time of manufacture. SAS drives are also hot pluggable and incorporate self-termination by default.

Predictions indicate that by the end of 2005, the full transition from parallel SCSI and Fibre Channel will place serial attached SCSI as the fastest growing storage technology for the industry.

The Technical Committee of the InterNational Committee on Information Technology Standards (INCITS, pronounced "insights") that works through SCSI standards is referred to as T10. It is operated by and accredited through the American National Standards Institute. For more in depth information about SAS, SATA and SCSI drive technologies and standards, working drafts can be found at Information gathered for this installment was provided in part by HP, the SCSI Trade Association, Seagate, Adaptec and INCITS.