Serial Attached SCSI

SAS

SAS protocols, physical links and ports: what an expander or wider connection does, and what it does not guarantee.

Glossary entry on hesela.dev · JSON record · Serial SCSI Protocol · Serial ATA Tunneled Protocol · Serial Management Protocol · SAS physical PHY · SAS wide port

In one minute

Serial Attached SCSI is a serial storage interconnect with distinct protocols for SCSI traffic, SATA tunneling and domain management.

SAS separates several questions that a connector label cannot answer: which protocol carries a command, which PHY carries a connection, and which paths reach the target. SCSI semantics can be carried over SAS without making the command set and the physical interface the same thing. [1]

SSP carries SCSI exchanges. STP carries SATA communication through the SAS portion of an expander path, with SATA on the final device link. SMP handles management exchanges such as discovery. These are protocol roles, not three recording technologies. [2]

Tunneling is not command translation. SCSI / ATA Translation (SAT) maps between command models; STP provides transport. Calling an expander a SCSI-to-ATA translator merely because it supports STP conflates those layers. [6]

A connector is not necessarily one logical SAS port. Four PHYs in a connector can form one wide port or separate ports, depending on the attachments. Likewise, a dual-port drive can have two narrow ports: port count and port width answer different questions. [4]

A wide port permits separate connections, while an individual connection uses one pathway. A wider upstream port therefore does not automatically multiply the throughput of a single narrow-port drive. Check both endpoints and the shared path before inferring capacity. [3]

SAS is not synonymous with 12 Gb/s. Physical-interface and protocol-layer specifications evolve separately: T10 lists SAS-4.1 as INCITS 567-2023 and SPL-5 as INCITS 554-2023. Identify the hardware generation and negotiated link rate instead of treating a familiar product-era number as a universal SAS property. [8]

Compared to neighbors

Compare protocol, topology and workload separately. A SAS label does not certify a carrier's vibration tolerance or determine platter speed; those require device and enclosure evidence.

SAS layers and topology: separate the role from the guarantee.
Concept	What it identifies	What it does not establish
SSP	SCSI transport [1]	Media durability from transport alone
STP	SATA tunneling [1]	SCSI-to-ATA translation [6]
SMP	Management exchanges [2]	User-data integrity
Physical PHY	One physical-link endpoint [5]	An entire connector or port
Wide port	Multiple PHYs in one port [3]	Linear speedup for one narrow target
Dual-port device	Two device ports [4]	Independent end-to-end failure domains

Strengths

Expander fan-out - an initiator can reach multiple targets through routed connections, rather than needing one direct cable per target. [5]
Path diversity - suitably wired dual-port devices can offer more than one route to the same storage. [5]
SATA attachment - compatible SAS controllers and connections can accommodate SATA drives; compatibility in the opposite direction does not follow. [7]

Limits

Shared resources - upstream links, controller capacity and endpoint behavior can limit aggregate throughput; port width alone is not a performance measurement.
Compatibility is conditional - a SAS drive cannot operate on a SATA-only controller, and cable fit alone does not establish support. [7]
Availability is a system property - two drive ports do not remove shared controller, expander, power or enclosure failure domains.

Typical fits

Shared disk shelves behind SAS expanders
Storage with explicitly designed redundant controller-to-drive paths
Mixed SAS/SATA attachments supported by the actual controller and enclosure

Uncertainty notes

This is an architectural explanation, not a bandwidth benchmark or availability measurement.
The 2002 T10 presentation and 2009/2010 guides support foundational distinctions, not current topology-size limits or universal implementation behavior.
Standard identifiers were checked on 2026-10-17. A published standard is not evidence that every installed controller implements it.

Sources

01SNIA dictionary: SAS Protocol Layer · accessed 2026-10-17
02T10/02-158r1: SAS technical overview, slides 14, 87, 89 and 93-97 (2002) · accessed 2026-10-17
03Seagate SAS Interface Manual, Rev. C (2009), sections 2.1.2, 2.1.4 and 5.11.1-2 · accessed 2026-10-17
04Oracle HBA guide: SAS phys, ports, addresses and connectors (2010), appendix G · accessed 2026-10-17
05IBM Power6 documentation: SAS architecture · accessed 2026-10-17
06T10 working drafts: SCSI / ATA Translation scope · accessed 2026-10-17
07Seagate: Serial-Attached SCSI installation instructions · accessed 2026-10-17
08T10 published standards: SAS-4.1 and SPL-5 · accessed 2026-10-17