Acronym · Memory & Storage
SAS (Serial Attached SCSI)
The enterprise serial storage interface carrying the SCSI command set, supporting dual-port drives, expanders and long cable runs at 12 Gb/s or, as 24G SAS, 22.5 Gb/s.
It is still widely used in large disk and hybrid arrays.
- Type
- Acronym
- Domain
- Memory & Storage, Storage interface
Notes
Dual porting for path redundancy is the feature that kept SAS alive in arrays long after NVMe won on performance.
Overview
SAS, or Serial Attached SCSI, is a point-to-point serial interface for connecting hard drives and SSDs to servers and storage systems, carrying the SCSI command set over high-speed serial links. It is defined by the INCITS T10 committee, whose published versions run from SAS-1.1 to SAS-4.1, with SAS-5 approved but not yet published. SAS is built for enterprise storage: it supports expanders that connect many drives to one controller, dual-ported drives for redundant paths, and SATA drives on the same backplane.
How it works
A SAS domain links initiators, typically host bus adapters or RAID controllers, to targets, the drives. Three protocols share the links: the Serial SCSI Protocol (SSP) carries SCSI commands to SAS drives, the Serial ATA Tunnelling Protocol (STP) carries commands to SATA drives through the SAS fabric, and the Serial Management Protocol (SMP) configures expanders. Expanders work rather like network switches, letting one controller reach many drives and supporting zoning so that hosts see only the drives assigned to them. Narrow ports use one lane; wide ports bundle several lanes for more bandwidth.
SAS drives have two ports, so a drive can connect to two controllers or two expander paths and stay reachable if one fails. This dual-domain arrangement suits high-availability storage. SATA drives can plug into SAS backplanes, though not the other way round.
Each generation has raised the link rate: 3, 6 and 12 Gb/s, then SAS-4, marketed as 24G SAS. SAS-4 moved to 128b/130b encoding, which Broadcom says doubles the effective link bandwidth, and added forward error correction to keep data reliable at the higher speed. A 24G expander can still run older 12G and 6G SAS drives and 6 Gb/s SATA drives, and some aggregate their slower traffic so the host links run at full speed.
Examples
- Broadcom's SAS4xNN expander family, offering 24 to 48 ports for 24G, 12G and 6G SAS and 6 Gb/s SATA devices, with zoning and hardware secure boot.
- A high-availability array in which each dual-ported SAS drive is wired to two controllers, so I/O continues if one controller fails.
- A JBOD enclosure where cascaded expanders let one host bus adapter reach dozens of nearline SAS hard drives.
- A server backplane accepting both SAS and SATA drives, with SATA traffic carried over STP.
Why it matters
Drive enclosures, external RAID systems, JBODs and server backplanes are built around SAS, especially for hard drives. Storage controller and expander chip designers implement the protocol and PHYs, verification engineers test interoperability against the T10 standards, and storage system architects build redundant topologies on top of it.
In the chip lifecycle, SAS PHY, link and protocol layers are specified at architecture against a particular SAS and SPL version, verified with compliance and interoperability testing before and after tapeout, qualified with many drive models, and supported in storage products for long service lives.
It matters most to: Enterprise Storage Systems Architect, Enterprise Storage Systems Engineer, Storage Benchmarking & Validation Engineer, Storage Controller RTL Engineer, Storage Controller Verification Engineer
Common confusions
- Not the same as SATA
- SATA is the simpler, lower-cost serial interface used in PCs, with single-ported drives and shorter cables. SAS hosts can run SATA drives, but SATA hosts cannot run SAS drives. SAS vs SATA
- Not the same as NVMe
- NVMe is a command set designed for flash that runs directly over PCI Express. SAS carries the older SCSI command set over its own serial links and remains common for hard drives.
- Not the same as SAS the analytics software
- SAS is also the name of a well-known statistics and analytics software company, unrelated to storage interfaces.
Related standards
- INCITS 567-2023 (SAS-4.1, Serial Attached SCSI physical and link layers)
- INCITS 554-2023 (SPL-5, SAS protocol layer)
Sources
- T10 Working Drafts and Standards status list (opens in a new tab), INCITS T10 Technical Committee, 2026-10-22
- SAS4xNN Series: Pin-Compatible SAS/SATA Expander Series Featuring SAS-4 Technology (product brief) (opens in a new tab), Broadcom, 2026-10-22
- Serial Attached SCSI (Wikipedia) (opens in a new tab), Wikimedia Foundation, 2026-10-22
Related terms
Part of the Memory & Storage glossary, with every term in the topic and the lifecycles, tools and standards that use them.
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