
NAND flash memory die

A group of Japanese researchers led by Ken Takeuchi, a professor at Chuo University, has announced a hybrid SSD architecture that combines a ReRAM buffer with standard NAND flash. The resulting hybrid offers significantly improved performance and longevity, and could be a model for manufacturers trying to overcome some of flash’s scaling problems.

At present, long-term NAND scaling faces a number of problems. Flash built on smaller process nodes is less durable. Attempts to move from MLC (two bits per cell) to TLC (three bits per cell) have been slow thanks to the drastically reduced number of write cycles TLC flash can perform; QLC flash (four bits per cell) is wishful thinking. In theory, a ReRAM hybrid could solve a number of these problems.

ReRAM (Resistive Random-Access Memory) is one of a number of technologies under development by companies looking for next-generation storage mediums to replace NAND flash. In this context, its chief advantage over flash is that it’s both more durable and requires far less energy per write.

The research team proposes a hybrid drive that combines 256GB of NAND with ~1GB of ReRAM for both cache and storage. Instead of writing directly to the NAND, drive writes would be cached within ReRAM until a certain threshold was reached. These would then be written in bulk, thereby lowering data fragmentation and drastically reducing the number of write cycles the NAND flash performs.

SSD data comparison

Data is buffered and handled according to three algorithms. First, an anti-fragmentation filter ensures that writes are stored until they reach at least 60% of 16K depending on the ReRAM’s capacity. Second, data that’s overwritten in NAND is copied back to the buffer (to prevent fragmentation and additional re-write cycles). Finally, the buffer keeps a record of the most recently accessed addresses. If data is being written and re-written rapidly, the updates occur in ReRAM, not the NAND.

Hybrid SSD

I’m normally extremely dubious of watershed innovations in technology, but one of the most striking features of Takeuchi’s proposal is that the amount of ReRAM required is quite small compared to the storage capacity of the drive. That’s important when considering the impact of economies of scale. Manufacturers who are currently building out NAND fab capacity aren’t going to fall all over themselves to retool for ReRAM (or anything else) unless they absolutely have to. Incorporating a small amount of ReRAM around high-capacity NAND makes much more sense and preserves the value of the current investment.

Even better, there’s the chance that the ReRAM buffer’s cost could be offset by the use of TLC flash at some point in the future. The researchers designed their prototype by assuming the use of TSVs (through-silicon vias) but believe the design could be adapted to other approaches currently in mass production. TSVs have been demoed and are under development, but aren’t yet shipping in widespread numbers.

The ReRAM buffer technique doesn’t solve the long-term need for a NAND replacement, but it would be a way to implement early manufacturing while research continues. The team has yet to build a physical prototype of their design, so current conclusions, while positive, should be taken with a grain of salt.

Even if the ReRAM approach doesn’t bear fruit, we expect to see more of these sort of buffers in the future. In theory, drive manufacturers could create specialized blocks of reliable (and expensive) SLC flash cache surrounded by an MLC or TLC main drive. While the entire drive would still use NAND, rapid writes would first be performed on the SLC block, where its much higher write endurance would provide a similar benefit. The lower-durability TLC/MLC flash would be treated as a “Read/Write Occasionally” data storage area.

The types of solutions we’ll see in the next 3-5 years will depend on how rapidly NAND alternatives mature and how successfully companies can deploy future process nodes. Such drives could take a few years to trickle down into consumer hands depending on cost scaling and enterprise suitability. Buffer drives could conceivably form a fourth tier of storage alongside cache drive SSDs, standalone SSDs, and HHD (hybrid hard drives). At present, analysts believe that the cache drive + HDD solutions will skyrocket in popularity over the next 12-18 months. Buffered drives could offset the reliability concerns that drive the HDD + cache drive deployments and reassure customers that they only need one product — not two.