This year will be big—and fast—when it comes to storage. SCSI attached storage (SAS) is moving to 12-Gbit/s rates, and flash technology has pushed past this transfer rate. Emerging standards like NVM Express, SCSI Express, and SATA Express look to bring faster storage operation to PCI Express-based platforms.
PMC-Sierra and LSI are addressing the 12-Gbit/s SAS market with SAS RAID-on-chip (ROC) and SAS Expanders. Also, PMC-Sierra has a single-chip, 68-port SAS Expander. I expect 12-Gbit/s SAS hard-disk and flash drives to be available this year.
SATA drives may follow suit, but 6-Gbit/s drives are likely to be the mainstay for consumers and most commercial users with large enterprise systems employing 12-Gbit/s technology. Hybrid technology like Seagate’s Momentus XT will handle single-drive consumer applications.
Micron’s RealSSD P320h (Fig. 1) and Fusion-io’s ioDrive (Fig. 2) plug into PCI Express Gen 2 sockets to deliver more throughput than is possible using SATA or SAS interfaces. Each uses proprietary drivers, but the industry is moving toward standard interfaces (see “NVM Express: Flash At PCI Express Speeds”).
The NVMHCI Work Group delivered the NVM Express (NVMe) standard last year. With the advantage of being out the chute first, it specifically targets flash storage and boards such as the P320h and the ioDrive Octal.
Like SCSI and SATA Express, NVMe presents a standard PCI Express hardware interface, enabling a single generic driver to handle various hardware incarnations. This is already the case for PC devices like integrated device electronics (IDE) and USB hub interfaces.
SCSI Express and SCSI over PCIe (SOP) are designed to deliver a standard PCI Express-based SCSI controller interface, essentially unifying the SAS controller market. SAS differs from NVMe because it can support any type of storage, although flash and hard drives are the dominant platforms.
NVMe and SCSI Express might come together since the two queueing mechanisms for SCSI Express include NVMe and PCIe Queuing Interface (PQI).
SCSI Express uses the SCSI command set. A controller could be the front end for a single drive or an entire array. SAS has significant advantages when it comes to enterprise and server storage management.
Already, SAS handles hot swapping and array management. HP is one of the driving forces behind SCSI Express. The T10 Technical Committee is responsible for SCSI and SAS standards.
The Serial ATA International Organization (SATA-IO) is taking a similar approach to SCSI Express with SATA Express. Of course, SATA Express utilizes the SATA command set, enabling the interface to be a front end for existing SATA drives as well as on-board flash.
SATA Express looks to boost throughput to 8 Gbits/s and 16 Gbits/s targeting flash technology. The standard will define device and motherboard connectors
From Drives To Modules
The interface standards only care about PCI Express, so they can be applied to any existing PCI Express platform, from PCs to VPX systems. Still, standard hardware form factors are advantageous to modular systems.
PCI Express modules like ExpressModule already exist for servers. The ExpressModule uses edge connectors that aren’t as rugged as other connector technologies.
Several PCI Express-based connection technologies are in the works. The SFF Committee is an ad hoc group that targets storage interfaces. The SSD Form Factor Working Group is also looking at an interface solution that would unify SAS, SATA, and PCI Express based on the 2.5-in. drive form factor. If we’re lucky, there will be a single interface standard in the near future.
The standards are still a work in progress, but everything probably will come together this year. Regardless, PCI Express remains the common factor.