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SCSI trade group claims new SAS has pluses over NVMe/PCIe

NVMe and PCIe solid-state drives (SSDs) may be a hot topic at this week’s Flash Memory Summit, but the SCSI Trade Association is trying to remind everyone that new serial-attached SCSI (SAS) technology is on the way.

Rick Kutcipal, president of the SCSI Trade Association and product planner at Broadcom, said he expects the upcoming “24 Gigabits per second” (Gbps) SAS device-connect technology – which actually has a maximum bandwidth of 19.2 Gbps – to see its first use with SSDs.

“The biggest advantages will be in solid-state memory,” Kutcipal said.

He said the SCSI Trade Association hopes to hold its first plugfest for so-called “24 Gbps” SAS in mid-2017. He expects host bus adapters (HBAs), RAID cards, and expanders to support the new SAS technology in 2018, with server OEM products to follow in 2019.

Kutcipal claimed the 19.2 Gbps bandwidth would have a 21.5% per-lane performance advantage over non-volatile memory express (NVMe) running on top of PCI Express (PCIe) 4.0. The maximum bandwidth for single-lane PCIe 4.0 is 15.8 Gbps, he said.

SAS typically uses one lane to the drive, and enterprise NVMe SSDs typically use four-lane PCIe, Kutcipal acknowledged. Four-lane PCIe would obviously be faster than single-lane SAS.

But Kutcipal said, “The lanes are not free. [They’re] actually very expensive, so the comparison has to be per lane. SAS can go x2 or x4 [lanes] to the drive. ”

SAS uses the small computer system interface (SCSI) command set to transfer data between a host and a target storage device. SCSI was developed 30 years ago when hard disk drives (HDDs) and tape were the primary enterprise storage media. Manufacturers have continued to use serial-attached SCSI as a drive-connect with faster SSDs.

The SCSI Trade Association’s efforts to promote a new SCSI Express (SCSIe) interface to run SCSI commands over PCIe have largely fallen flat in comparison to the momentum behind NVMe with PCIe-based SSDs.

The NVM Express industry consortium developed NVMe as a lower-latency alternative to SCSI. NVMe streamlines the register interface and command set for use with faster PCIe-based SSDs and post-flash technologies, such as Intel-Micron’s 3D XPoint.

“SAS is inherently scalable, and NVMe is not,” Kutcipal said. “NVMe will scale to tens of devices, and it’s pretty arduous scaling, while SAS can go to thousands of devices. And there are arrays out there today that are thousands of devices.”

Kutcipal said NVMe cannot solve PCIe’s scaling challenges.

“The limitation in the scalability of NVMe as a device connect is really inherent in PCIe, not in NVMe,”he said. “That’s a big fundamental limitation of NVMe. It relies on PCI Express as its transport in the device connect world.”

SAS can serve as a device/drive connect as well as a storage networking technology. But Kutcipal said the dominant role for SAS is connecting a host bus adapter (HBA) or RAID card to an SSD or hard disk drive (HDD). SAS has distance limitations for storage networking, limiting its use to SANs inside the data center, he said.

The upcoming SAS specification has two parts: the SAS-4 physical layer and the SAS Protocol Layer (SPL)-4. The SPL-4 specification is expected to be complete and ready for use later this year, according to Kutcipal. He said SAS-4 would lag SPL-4 by a quarter.

In addition to the speed bump, new features on the way with next-generation SAS include Forward Error Correction, to ensure data integrity, and continuous adaptation, to enable the SAS transmitter to operate optimally, even if the temperature or operating voltage changes, Kutcipal said.

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