Partnership Puts ReRAM in SSDs

　　Solid state drives (SSDs) are pretty much synonymous with NAND flash, but there have been attempts to use a different persistent memory with varying degrees of success.

　　Mobiveil Inc. and Crossbar Inc. recently announced they are collaborating to use resistive random access memory (ReRAM) in an SSD. The collaboration will apply Mobiveil's NVMe SSD IP to Crossbar's ReRAM IP blocks. The goal is to enable 10 times more IOPs at one-tenth of the latencies of flash NVME SSDs to speed up access to frequently requested information in large data centers, the companies told EE Times in a joint telephone interview.

　　Mobiveil CEO Ravi Thummarukudy said the company's NVMe, PCIe and DDR3/4 controllers can easily be adapted to accommodate the Crossbar ReRAM architecture, which is capable of six-million 512B IOPS below 10us latency. He said Mobiveil's NVM Express Controller architecture is designed to optimize link and throughput utilization, latency, reliability, power consumption and silicon footprint, and can be used along with its PCI Express (PCIe) controller and Crossbar's ReRAM controller.

　　Mobiveil's NVMe controller IP is outfitted with an AXI interface that simplifies integration with FPGAs and SoCs. Other IP subsystem components include PCIe Gen 3.0, DDR3/4, and ONFI controllers. An FPGA development platform includes BSPs and drivers for validating the NVMe IP solution against user applications. Crossbar ReRAM technology, meanwhile, can be integrated in standard 40 nm CMOS logic or produced as a standalone memory chip.

　　Thummarukudy said there's been a heightened level of interest in persistent memory since Intel and Micron introduced 3D Xpoint. Mobiveil and Crossbar have been working together for the last year, said Sylvain Dubois, vice president of strategic marketing and business development at Crossbar, and in addition to developing IP for ReRAM-based SSDs, they are also working on incorporating ReRAM into NV-DIMMs. “The NV-DIMM is the natural evolution of what we're doing with the SSD," Dubois said.

　　The key benefit of using ReRAM in an SSD is that it reduces storage controller complexity by removing large portions of the background memory accesses needed for garbage collection. It also provides independent, atomic erasure by eliminating the need to build large-block memory arrays in flash designs.

　　Neither Mobiveil nor Crossbar are building actual SSDs or NV-DIMMs. Rather, they have developed the IP so others can build their own solutions, said Thummarukudy.

　　The Mobiveil/Crossbar collaboration is not the first attempt to make SSDs out of something other NAND flash, said Jim Handy, principal analyst at Objective Analysis, including failed attempts to make DRAM-based SSDs. This effort seems similar to Intel's Optane offering, he said. Handy added that Intel and Micron are adamant that Optane doesn't use phase change memory (PCM), considered a subset of ReRAM.

　　Intel Optane is the only 3D Xpoint product to date, said Handy, and its chips are similar to that of Crossbar's ReRAM. “The chips themselves are both close to DRAM speeds but persistent," Handy said. "You put them behind an NVMe controller and its ends up being very fast."

　　Handy said the biggest disappointment around 3D Xpoint is that Micron and Intel were promoting speeds a thousand times faster than that of NAND flash, but in reality it's only been seven or eight times faster.

　　Handy expects Mobiveil and Crossbar's ReRAM-based SSD will suffer the same fate. “What's happening is the NVMe interface becomes a large part of the delay," he said.

　　When Intel pushed the NVMe specification, it made sure to put in hooks that would support 3D Xpoint well before the company even announced it had 3D Xpoint. “That was definitely in the back of their minds," Handy said. "They weren't just optimizing an interface for NAND flash."

　　Although there are customers who might want to build a ReRAM-based SSD, it's a small market, said Handy. “It's going to be pretty expensive," he added, saying its not dissimilar to the NV-DIMM market.

　　“The current NV-DIMMS are more expensive than DRAM, they're way more expensive than SSDs, but offer blazing speed for people who want to pay for it," Handy said.

　　Handy said the Crossbar ReRAM-based SSDs will find a niche with customers willing to pay top dollar for persistence and performance, adding that Intel is selling Optane at a loss because it helps the company sell more expensive processors.

　　SSDs with 3D NAND are not in any danger, said Handy. “They will be far more economical than anything made of out Crossbar ReRAM," he said.