Definition

SAS SSD (Serial-Attached SCSI solid-state drive)

Contributor(s): James Alan Miller and Carol Sliwa

A SAS SSD (Serial-Attached SCSI solid-state drive) is a NAND flash-based storage or caching device designed to fit in the same slot as a hard disk drive (HDD) and use the SAS interface to connect to the host computer.

The most common drive form factors for a SAS SSD are 2.5-inch and 3.5-inch. SAS SSD bandwidth options include 3 Gbps, 6 Gbps and 12 Gbps.

SAS SSDs are primarily used in enterprise servers and storage arrays with application workloads requiring high availability (HA), high input/output (I/O) and low latency. Use cases for SAS SSDs include server virtualization, online transaction processing, high-performance computing and data analytics.

Drive manufacturers sometimes offer SAS SSDs with different write endurance options. For instance, a high-capacity SAS SSD intended for read-intensive workloads might guarantee only one drive write per day (DWPD), while a lower-capacity SAS SSD intended for write-intensive workloads might support up to 25 DWPD.

SAS SSD vs. SATA SSD

A SAS SSD delivers faster data transfer rates than a serial ATA (SATA) SSD. In contrast to a SATA SSD, a SAS SSD also supports dual-port operation and builds in features to improve reliability, such as advanced error correction, data integrity technology and high signal quality on the cable or backplane.

SAS drives use a higher signal voltage than SATA drives and can reliably transmit data -- with better overall data integrity end to end -- at twice the speed of SATA drives. In addition, SAS drives -- unlike their SATA counterparts -- support multiple data paths, superior data-path redundancy and high availability.


This Techquickie video explains
the differences between SAS
and SATA.

You can use both SAS and SATA drives with a SAS interface in a server. Servers with ATA backplanes only support SATA drives, however. In addition, a SAS backplane will alter the signal when a SATA drive is plugged in. That way, the server's disk controllers will handle the SATA drive as if it were a SAS drive. This is an effort to boost SATA reliability.

Generally, enterprises buy speedy and reliable SAS SSDs for higher tiers of data and SATA drives -- with their much slower speeds and low cost per terabyte -- for capacity and lower-tier storage purposes. While SAS SSDs have traditionally been much more expensive than SATA SSDs, the price differences have been narrowing.

SAS SSD vs. PCIe

Peripheral Component Interconnect Express (PCIe) is a serial bus expansion standard that allows the attachment of peripherals directly to a computer. Unlike SAS SSDs, PCIe SSDs -- including those based on the emerging nonvolatile memory express (NVMe) specification that accelerates performance -- integrate flash storage right onto the server motherboard.

Because each PCIe SSD links to a host through its own serial link, the need to share a bus is eliminated, reducing latency and boosting data transfer rates between servers and storage arrays. This allows PCIe SSDs to sidestep the bottlenecks often associated with SAS- or SATA-attached SSDs.

SAS SSD vs. near-line SAS

Near-line SAS drives are essentially enterprise-class SATA drives with a SAS connector and a fully capable SAS interface command set. They offer many of the advantages of SAS drives -- support for command queuing, concurrent data channels and multiple hosts, for example -- and are slightly faster than SATA drives.

While sharing many SAS characteristics, near-line SAS drives are not SAS. They are targeted at NAS servers and store occasionally accessed data that is not considered mission-critical -- the opposite of SAS.

Pros and cons

SAS SSDs are fast, up to 12 Gbps, and require little hardware overhead. The latter is important when it comes to maximizing IOPS and reducing data latency. They deliver a high level of end-to-end data integrity and include features like error correction to improve reliability.

SAS drives come in a variety of endurance options, depending on the manufacturer, to match particular workload and write requirements. Support for multiple data paths makes for higher levels of performance and availability, while also enabling data path redundancy.

SAS SSDs are, by their nature, all about performance and reliability. Organizations would not purchase these drives purely for capacity purposes. SAS SDs also tend to be pricier than their SATA counterparts.

The future of SAS SSDs

SAS SSDs using the next-generation SAS 4.0 specification are on the horizon. The 4.0 specification also increases the full throughput per lane to a nominal 24 Gbps -- the actual maximum throughput is 22.5 Gbps -- a huge jump over the 12 Gbps maximum bandwidth for SAS 3.0.

Version 4.0 of the spec is composed of two parts: the SAS-4 physical layer and the SAS Protocol Layer (SPL)-4. SAS-4 will lag behind SPL-4 by approximately a quarter when the former is finally released.

According to the SCSI Trade Association, we will see RAID cards, expanders and host bus adapters that support the new SAS technology in 2018. OEM products will most likely follow in 2019.

In addition to an increase of speed, next-generation SAS will include forward error correction to ensure data integrity. A feature called continuous adaptation will allow a SAS transmitter to work optimally even if the temperature or operating voltage fluctuates.

This was last updated in December 2017

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