Serial Attached SCSI
Serial Attached SCSI (SAS) is a point-to-point serial protocol designed for high-speed data transfer between computers and storage devices, such as hard disk drives and solid-state drives, serving as an evolutionary successor to parallel SCSI with enhanced performance, scalability, and reliability for enterprise applications.[1] Developed by the T10 technical committee under ANSI/INCITS, SAS was standardized in 2003 (INCITS 376-2003), with products introduced in 2004, building on SCSI commands while adopting serial transmission to overcome limitations like cable length and device count in parallel interfaces.[2] It utilizes a layered architecture, including physical, link, transport, and application layers, with data rates starting at 3 Gbit/s and advancing to 22.5 Gbit/s in recent generations.[3] At its core, SAS employs three key transport protocols: the Serial SCSI Protocol (SSP) for direct SCSI command execution, the Serial ATA Tunneling Protocol (STP) to enable compatibility with SATA drives on the same infrastructure, and the Serial Management Protocol (SMP) for configuring and monitoring expanders that extend connectivity.[2] This dual support for SCSI and SATA devices allows seamless integration in mixed environments, reducing costs and simplifying deployment in data centers.[1] SAS domains can scale to support up to 65,536 devices through cascaded expanders, each handling up to 128 connections, making it ideal for mission-critical storage arrays requiring high availability and fault tolerance.[4] The protocol's latest iteration, SAS Protocol Layer 5 (SPL-5) under INCITS 554-2023, further refines serial interconnect rules for improved efficiency in modern peripherals like tape drives and SSDs, with ongoing work on 24G+ enhancements and SAS-5.[3][5]Overview
Definition and Purpose
Serial Attached SCSI (SAS) is a point-to-point serial protocol that leverages the SCSI command set to enable data transfer between computer systems and storage devices, including hard disk drives (HDDs), solid-state drives (SSDs), and tape drives.[4][6] This interface uses thin serial cables to establish direct connections, facilitating reliable and efficient communication in storage-intensive applications.[4] The purpose of SAS is to deliver high-performance, scalable connectivity for storage in enterprise environments, such as data centers, servers, and storage area networks (SANs). It supports up to 16,384 devices per domain, enabling expansive configurations that meet the demands of large-scale data management and archiving.[1] Key components of SAS include initiators, which are host bus adapters or controllers that send commands; targets, the peripheral storage devices that receive and process those commands; expanders, which act as intelligent switches to route connections and extend the network; and service delivery subsystems, encompassing the physical infrastructure like cables and backplanes that link these elements.[4][6] SAS emerged as the successor to parallel SCSI, overcoming its predecessor's constraints on cable length and the number of attachable devices.[4][7] It maintains compatibility with the SCSI command set, ensuring broad interoperability with established storage protocols.[6]Key Features and Advantages
Serial Attached SCSI (SAS) supports dual-port architecture, allowing each device to have two independent ports for enhanced redundancy and failover capabilities in high-availability environments.[8] This dual-port design ensures that if one path fails, the other can seamlessly take over, minimizing downtime in enterprise storage systems.[9] SAS incorporates zoning and port multiplication through its expander devices, enabling logical isolation of storage resources and improved scalability. Zoning, standardized in the SAS-2 specification, allows administrators to create secure zones that control access between hosts and targets, similar to Fibre Channel zoning, supporting up to 256 devices per expander for denial-of-service protection and device isolation.[10] Port multiplication in expanders permits a single physical port to function as multiple logical ports, expanding connectivity without additional controller ports and facilitating large-scale deployments.[11] The protocol operates in full-duplex mode, enabling simultaneous bidirectional data transfer, and supports wide ports comprising up to four lanes per port to aggregate bandwidth for higher throughput.[8] Additionally, SAS is compatible with SATA devices through the SATA Tunneling Protocol (STP), which encapsulates SATA commands within SAS frames, allowing seamless integration of SATA drives into SAS domains for mixed-environment flexibility.[8] SAS offers superior reliability via robust error detection and correction mechanisms, including cyclic redundancy checks (CRC) on all frames to ensure data integrity during transmission.[8] It supports cable lengths up to 10 meters for external copper connections, providing greater deployment flexibility compared to shorter alternatives.[12] Furthermore, the protocol accommodates multiple initiators accessing a single target, enhancing resource sharing in multi-host configurations.[8]History and Standards
Development and Evolution
The development of Serial Attached SCSI (SAS) originated in the early 2000s as an evolutionary successor to parallel SCSI, addressing key limitations such as restricted device counts per bus (typically 15 devices) and short cable lengths due to signal skew and noise in parallel transmission.[1] In May 2002, the INCITS Technical Committee T10, responsible for SCSI standards, accepted a project proposal from industry leaders including Dell, HP, and Intel to create a serial interface that retained SCSI's command set while enabling point-to-point connections for improved scalability.[13] The SCSI Trade Association (STA), formed to promote SCSI technologies, collaborated closely with T10 to drive this initiative, culminating in the first ANSI-approved SAS standard, INCITS 376-2003, published in 2003.[14][15] Key milestones marked SAS's progression from specification to market reality. The first SAS-compliant products, including host bus adapters and drives from vendors like Seagate and LSI Logic, became available in 2004, with demonstrations of functional silicon occurring as early as January of that year.[1][16] By 2009, SAS achieved widespread adoption in enterprise environments, driven by the release of enhanced standards that supported broader deployment in servers and storage arrays.[17] Post-2010, SAS integrated seamlessly with solid-state drives (SSDs), with major announcements such as Seagate's Pulsar SAS SSDs in 2011 and subsequent entries from Samsung and Micron, enabling high-performance flash storage in data centers.[18] The primary drivers for SAS's evolution were the demand for higher data transfer rates beyond parallel SCSI's Ultra320 limits, enhanced scalability to support thousands of devices through switched topologies, and the industry shift toward serial interfaces for reduced complexity and better reliability in expanding storage ecosystems. ANSI-accredited INCITS, through its T10 committee, handled the core technical specifications and protocol definitions, ensuring compatibility and interoperability.[19] Meanwhile, the STA—now operating under the Storage Networking Industry Association (SNIA)—played a pivotal role in promotion, education, and roadmap development to accelerate industry adoption.[20][14]Generations and Speeds
Serial Attached SCSI (SAS) has progressed through multiple generations, each advancing data transfer rates while incorporating encoding improvements to enhance efficiency and reduce overhead. The initial generations relied on 8b/10b encoding, which maps 8 bits of data to 10-bit symbols for DC balance and clock recovery, resulting in 20% overhead. Later generations shifted to more efficient schemes like 128b/130b encoding, which prepends a 2-bit sync header to 128 data bits, yielding approximately 98.5% efficiency. SAS-1, standardized as INCITS 376-2003 and introduced in products by 2004, operates at 3 Gbit/s per lane using 8b/10b encoding as its basic serial implementation. This provides an effective throughput of about 300 MB/s after encoding overhead, marking the transition from parallel SCSI to serial point-to-point connections.[14] SAS-2, ratified as INCITS 457-2010 and available since 2009, doubles the speed to 6 Gbit/s per lane while retaining 8b/10b encoding. It introduces the SATA Tunneling Protocol (STP) to encapsulate SATA commands over SAS links, enabling compatibility with SATA devices, and includes improved power management for better energy efficiency in enterprise environments.[21][22] SAS-3, approved as INCITS 519-2014 and released in 2013, achieves 12 Gbit/s per lane by adopting 128b/130b encoding for greater efficiency. This scheme reduces overhead compared to 8b/10b, with the effective data rate calculated as \frac{128}{130} \times raw rate, approaching 11.8 Gbit/s while maintaining compatibility with prior topologies.[23]| Generation | Year | Speed (Gbit/s per lane) | Encoding | Key Improvements |
|---|---|---|---|---|
| SAS-1 | 2004 | 3 (effective ~2.4) | 8b/10b | Basic serial point-to-point links |
| SAS-2 | 2009 | 6 (effective ~4.8) | 8b/10b | STP for SATA compatibility, power management |
| SAS-3 | 2013 | 12 (effective ~11.8) | 128b/130b | Higher efficiency encoding |
| SAS-4 | 2017 | 22.5 (effective ~19.2) | 128b/130b + FEC (eff. 128b/150b) | FEC for reliability, tri-mode support with PCIe/SATA |
| SAS-5 | 2023 | 45 (target, signaling) | TBD | Protocol enhancements for hyperscale, low latency |