SAS is said to be dying, but it still represents the biggest chunk of the market, and is going to be around for a while yet...
You’ve no doubt heard it before. Serial Attached SCSI (SAS) is dying as a connected storage protocol in the data center. The simple truth is SAS remains the dominant storage interface today and will be for years to come.
When you look at the forecast by market analyst firm TrendFocus for enterprise storage capacity (Figure 1), you can see SAS will continue its strong growth through at least 2024. Yes, it’s true that nearline hard disk drives (HDDs) are the bulk of the forecast. And although the use of solid-state drives (SSDs) in enterprise storage is on the rise, nearline hard disk drive capacity makes up by far the vast majority of total capacity shipped and is only going to increase each year for the foreseeable future.
It should be noted that the PCIe interface is now the most common for SSDs in enterprise storage (Figure 2). The tipping point occurred in 2019 when PCIe NVMe based SSDs just out-shipped SAS and SATA SSDs combined for the year. The pendulum has swung, but even when looking at SSDs only, SAS and SATA combined still represent at least a quarter of the total flash capacity shipped through 2024. The bulk of those SATA SSDs are connected via SAS infrastructure.
SAS is the serialized version of the Small Computer System Interface (SCSI), which was first published as a standard in 1986. The organization that develops the SCSI/SAS standards is known as T10, a Technical Committee of the InterNational Committee for Information Technology Standards (INCITS). These SCSI/SAS standards define the protocol, command set, and physical transports to connect peripheral devices to a host, typically a server or storage system.
SAS continues to evolve, focusing on high performance, high reliability, scalability, flexibility, and manageability. Supporting SAS and SATA HDDs and SSDs, fabric-like connectivity, and a massive ecosystem that also includes RAID controllers, host bus adapters, cables, connectors, and expanders, SAS clearly spans the data center. It’s a mature and proven technology that has produced 11 generations of storage advancements, effectively doubling bandwidth performance and growing its capabilities with each generation.
SAS consists of several specifications (Figure 3) that define the physical, protocol, and SCSI command layers that generally align to generational releases such as 12Gb/s SAS and 24G SAS. Most commonly associated with these generational releases is the doubling of the effective bandwidth, which is covered in the physical layer specification. But also included in the physical layer are other key features, such as forward error correction (FEC) for the highest levels of data integrity.
The protocol layer of SAS evolves to support the physical layer, but also to integrate system-level features that enable use of new media and large-scale system requirements. One such feature introduced with 24G SAS is active PHY transmitter adjustment (APTA). APTA dynamically optimizes signal integrity, which in turn allows the higher bandwidth of 24G SAS without sacrificing the connectivity and cable distances supported in previous generations.
SAS cabling solutions, including optical variations, provide rack-to-rack, room-to-room, and data center-to-data center connection options. The SCSI Command layer evolves with storage media technologies to enable data management, device configurations, and error containment. With 24G SAS, this intersects with new media innovations geared toward capacity and performance optimizations for HDDs and SSDs, such as Shingled Magnetic Recording, MultiLink SASTM, and storage intelligence.
The scalability of SAS has been a major benefit for enterprise storage for decades, having been designed to accommodate configurations up to 65,536 (2^16) end devices. While topologies of that scale are not necessarily realistic in practice due to corresponding memory requirements, configurations of up to 2,000 devices have been deployed, typically as storage area networks (SANs).
A SAN often consists of multiple hosts and SAS-based storage that can be dynamically scaled by adding disks or arrays with zero downtime, a feature also known as hot plug. Hot swap support, similar to hot plug, is another critical aspect of enterprise storage management that has traditionally proven to be difficult to implement for newer interfaces, especially surprise hot swap, which occurs when a drive needs to be replaced unexpectedly even if the connection can’t be quiesced. SAS has supported both surprise hot swap and hot plug for generations.
Managing and maintaining storage can be challenging, especially for such large topologies, and it has taken decades of continual refinement to set the standard for enterprise data centers. The T10 Technical Committee responsible for SCSI storage interfaces created a specification called SCSI Enclosure Services (SES) which defines in-band and out-of-band (OOB) methods to communicate with SAS-based arrays and the devices inside them (e.g., fans, temperature sensors, status LEDs, and power supplies).
Applications can monitor enclosure services outside of the SAS connection with a Baseboard Management Controller (BMC) which reports the health and management status of the enclosure and devices via OOB communication, in case there’s a problem that would prevent the reporting of information through the SAS connection. All major enterprise OEMs have management tools that work with and support SES. It’s a well-understood solution to look “under the hood” and make sure things are running smoothly. In fact, SES has proven to be so robust and trusted that NVMe leverages much of SES for its own NVMe-MITM management specification.
Flexibility with SAS is inherent and built-in at the architectural layer. SAS connections and backplanes natively support both SAS and SATA protocols, as well as HDDs and SSDs, not to mention optical and tape drives. With a single SAS interface, you can create fully tiered environments that service high-performance applications with a caching layer (hot tier), warm tiers with a large amount of regularly accessed data, and an archival (cold) tier that uses low-cost, high-capacity HDDs.
Having proven itself through decades of hardening and maturity, SAS is the tried and true storage technology for data centers. SAS continues to innovate, focusing on high performance, high reliability, scalability, flexibility, and manageability to meet the ever-increasing demands of modern data centers. From cloud storage, social media, and the Internet of Things, to content delivery, artificial intelligence and machine learning, to augmented, virtual, mixed, and extended reality, SAS can meet the requirement. Ubiquitous in data center storage, SAS won’t cede its place any time soon.
Cameron Brett and Jeremiah Tussey are president and vice president, respectively, of the SCSI Trade Association. Brett is director of enterprise and cloud storage marketing at Kioxia. Tussey is alliances manager, product marketing at Microsemi Corp.