PCIe接口才是固態(tài)硬盤以后大勢所趨

Solid-state drives jump on PCI Express

Rick Merritt

SAN JOSE, Calif. – A wide group of vendors is poised to roll solid-state drives for the PCI Express bus, promising improved performance over current flash drives that mainly use serial ATA and Serial-Attached SCSI interfaces. The wave of new products will continue the current split between SATA- and SCSI-based protocols, and accelerate the shift to competition based on new software features.

As many as 80 companies including Dell, Intel, Micron, Oracle and Stec are part of the trade group that defined the NVMe interface last year. The first drives using the interface are expected to ship later this year.

Separately, the SCSI Trade Association (STA) recently announced it will adopt the SCSI over PCI Express standard being completed by the ANSI T10 committee. STA will hold a technology showcase in Silicon Valley on May 9 where members may demo some of the first SCSI Express flash drives.

The competing NVMe and SCSI Express drives are expected to continue the same split between SATA and SCSI command sets that exists in today’s SATA and SAS flash and hard-disk drives.

The NVMe and SCSI Express drives represent a challenge to Fusion-IO, a startup that soared to success based on pioneering the move to plugging solid-state drives into the fast PCIe bus, closely linked to system CPUs. Most first-generation solid-state drives used the slower SATA and SAS hard drive interfaces that reside lower in the hierarchy of interconnects on a server.

The performance benefits of PCIe helped Fusion IO tap into sales that soared to $84 million in its most recent quarter. The strong sales supported two successful public offerings in the past two years, raising more than $300 million.

With the advent of many more PCIe flash drives this year, Fusion faces a two-fold challenge. A wider supply of standard drives could help speed price decreases in the sector. In addition, the presence of more solid-state drives will likely narrow Fusion’s performance benefits based on its proprietary approach.

Fusion pioneered a method of giving host CPUs fast access memory stored on flash drives. The company has a proprietary approach for sharing with the host processors a map that describes where all the data on a flash drive physically resides, enabling write access at latencies of as little as 15 microseconds. Most drives maintain less comprehensive logical-to-physical translation maps on the drive controllers and thus have higher latencies.

The new NVMe and SCSI Express specs will enable vendors to leverage common software stacks for PCIe drives, lowering their costs and time-to-market.

“Today most PCIe flash drives include a proprietary driver and no industry software standard,” said Amber Huffman, a spokeswoman for the NVMe group and a senior principal storage engineer at Intel. With NVMe’s consistent feature set, “we expect to see faster time-to-market and broader adoption,” she said.

For its part, Fusion will back the SCSI Express approach because it is based on the work of a recognized standards group, the ANSI T10 committee. Access to the NVMe spec requires signing a legal document managed by Intel Corp., the group’s leader, said Gary Orenstein, vice president of products at Fusion IO.

“Is too early to tell what the shift to NVMe and SCSI Express will mean,” said Orenstein. “Some people think there could be a merging of the two efforts eventually,” he added.

To date, nearly three-quarters of Fusion’s sales have gone to three large customers, likely the top server makers—Dell, Hewlett-Packard and IBM. Dell has already signaled its support for NVMe in its latest servers, casting a shadow over at least one of Fusion’s big customers.

Shift to software

Value-added software is increasingly the secret sauce for maintaining flash drive prices. For its part, Fusion says software is now the key focus of its R&D efforts, and it acquired IO Turbine, a developer of storage virtualization software, in August for $65 million.

Fusion is already shipping the renamed ioTurbine software. It enables solid-state drives to be used as memory caches on a server running VMWare virtualization software. Fusion sees opportunities to roll software that enables other applications with its flash drives in areas such as database, enterprise search and social gaming.

In this way, Fusion may again be pioneering the direction for the rest of the flash drive market—a move to flash-enabled applications software. Long term, the industry still needs broader standards and support from operating systems for how flash storage can fit into the memory hierarchy.

The good news for all sides is it’s still early days for solid-state drives with plenty of growth seen ahead. International Data Corp. expects the market for PCIe-based flash drives to expand 85 percent on a compound basis from 2010-2015. Even the older market for slower SATA and SAS flash drives will grow a solid 56 percent over that period, IDC predicts.

PCIe-based flash drives came from virtually nowhere in 2009 to sales of 200 to 300 million units in 2011, according to various market researchers.

Long term, PCIe will supplant SATA and perhaps SAS as well, said Huffman.

SATA development ended at the current 6 Gbit/second generation with a SATA/Express merger planned as the next step. “SAS will have trouble getting beyond 12 Gbit/s—we definitely see PCI Express as the future,” Huffman said.

Interest in the NVMe spec is strong based on engineers adding about 20 errata to the spec to date, a sign products are in development. “We’re processing lots of clarifications, and that means people are using the spec,” she said.

The group has interoperability test labs set up at the University of New Hampshire and it is working on extensions to the spec. They include options for supporting multiple hosts and power-saving features.