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The Evolution of SAS

Brad Craig, Product Marketing Manager, Enterprise Storage Division, PMC-Sierra Inc.

Remember the good old days when parallel buses reigned supreme? Back when you brought home that ultra-fast, high capacity 2GB SCSI-3 hard drive for $500? When it would take two hours to get the 3”x 8” parallel cables to fit into the small gap between the hard drives and the power supply? Oh, how times have changed…

From Parallel to Serial
Parallel cables were a mainstay of data transfer for 20+ years. However, as data rates rose, parallel transmission began to drag down the increasingly fast processors and the peripheral technologies they were connecting. Serial technologies have become more and more prevalent, to the point where today very few devices require parallel connections.

Parallel data transmission requires that all data bits across a parallel set of wires be captured together. As data patterns change, so does the speed of the data bits as they travel across a cable (signal skew). The combination of skew, crosstalk and other signal integrity issues limits parallel SCSI buses to a maximum frequency of 80 MHz (320 MB/s).

Speed, though, is not the only problem. Parallel connections employ large, wide connectors, and as technologies trended smaller, there has been less and less space available on the devices for a connector. Other limitations of the SCSI bus included a limit of only 16 devices per bus.

Over time, it became apparent that parallel SCSI had run its course. All major interfaces eventually transitioned to point-to-point serial buses. The days of parallel busses are gone, but along with the higher frequencies and vastly improved throughput of serial technologies, other exciting improvements have been made.

Next Generation SCSI
Serial Attached SCSI (SAS) has replaced parallel attached SCSI as the interconnect technology of choice. For SCSI, the transition to SAS has provided many new opportunities for larger scaling and increased overall system performance. Data centers are growing increasingly more complex, driven by the demand to support rich data content, greater system throughput, and legislation to support data storage, protection recovery, and archiving. SAS provides a scalable pointto- point topology capable of addressing storage connectivity at many levels.

Rather than being limited to a set number of similar devices, the SAS architecture supports multiple hosts and large numbers of device attachments, accommodating systems ranging from small direct-attached disk drives to many thousands of disk drives through network attached controllers. SAS includes support for hot-plugging drives, and can support any mix of drive speeds and types within a single domain. Perhaps the two greatest strengths of SAS are the ability to incorporate low-cost SATA drives, and the ability to provide high bandwidth and excellent data throughput.

High Bandwidth, High Throughput Protocols
At the heart of the higher throughput and bandwidth provided by SAS are the three protocols that operate within SAS:

Serial SCSI Protocol (SSP) — Enables end-to-end connections between SAS (SCSI) disk drives, tape drives, etc.
Serial Management Protocol (SMP) — Enables configuration and management of the SAS domain
SATA Tunneling Protocol (STP) — Enables compatibility with SATA drives

SSP is the protocol that enables point-topoint connections for data transfers between SAS devices. An initiator establishes a connection from one end of a topology to a target at the other end of the topology before data transfer is initiated. Since data transfers are in packet form at the link speed, connection resources are only used for a short period of time before they are released for other devices to use. This ensures balanced throughput for all devices in a given topology. Additionally, multiple links can be aggregated into wide ports to provide higher aggregate bandwidth and enhance overall system throughput.

Expanders and controllers use SMP to configure and manage the SAS domain. The protocol operates in-band over the SAS links. SMP provides a simple framework for device discovery, and configuration setup, as well as reporting functionality for status logging.

STP is the key to enabling SATA support within a SAS topology. A SAS controller uses STP to establish a connection to a SATA drive. SAS expander devices are able to identify when a SATA device is attached to their ports, and will use STP to open a connection for a SATA device. Once a connection is open, native SATA frames flow across the connection directly from an initiator to a SATA device.

SATA Support
One of the most intriguing features of the SAS protocol is that SAS is designed to be compatible with SATA drives. SATA hard disks provide the highest capacity at the lowest cost-per-gigabyte of any storage media. Additionally, the use of a SATA Active/Active port selector to “dual-port” a SATA hard disk drive (HDD) enables the design of fully redundant storage architectures that provide greater system fault tolerance.

The trade-off for SATA is reduced performance and lower reliability than enterprise-class SAS and Fibre Channel (FC) drives. Where SATA drives are used for infrequently accessed data, near-line storage, or backup, RAID is commonly used to mitigate the reliability risks of SATA storage.

First generation controllers, expanders, and port multiplexers have allowed systems to successfully evolve from bus-based parallel SCSI to link-based serial SCSI with links that can run at 3Gbit/s. Expander devices can be added to scale the SAS topology up to 16,000 devices in a single domain. Successful interoperability events (plugfests) have been held to confirm compatibility between different vendor devices and systems, ensuring that SAS is enterprise ready.

The Future of SAS
SAS–2 is currently being drafted, and the future of SAS looks bright. The next generation will include a higher link rate, improved bandwidth utilization, and many features to improve the robustness and manageability of SAS topologies. For example, it will provide additional status and reporting information to facilitate additional diagnostic functions. This ensures that optimal system operation can be maintained. In the event of a fault, this status data can be used to identify, isolate, and analyze fault and error conditions.

The major improvements of SAS–2 include:

6Gbit/s SAS — Doubles the link rate and bandwidth
Multiplexing — Optimizes bandwidth by enabling two 3Gbit/s devices to be aggregated into a single 6Gbit/s link
Zoning — Enables partitioning of a domain into smaller sets of accessible devices
Self-configuring expander devices — Accelerates system initialization, and change detection
Diagnostics and robustness — Improves status reporting and error notification

At 6Gbit/s, second generation controllers are optimized to take full advantage of the 5Gbit/s per link speeds of PCIe 2.0. These SAS–2 improvements ensure that SAS systems are faster, provide better bandwidth utilization, are easier to manage, and enable unmatched system robustness.

SCSI technology has evolved into a new level of system performance and robustness, building on an already impressive history of adaptation. The future of SAS is no different; it will continue to grow to address the needs of the data centre and provide the innovation necessary to enable the next generation of technology.

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