A research group at Chuo University in Japan has developed a prototype SSD that uses a combination of 1GB ReRAM and 256GB NAND to deliver up to 11 times faster writing performance compared to current NAND-only SSDs. The new prototype works in a similar way to a hybrid SSD+HDD drive, but where the ReRAM is used to handle small write and overwrite operations and frequent read operations. The group is led by Ken Takeuchi, a professor at the Department of Electrical, Electronic and Communication Engineering at Chuo University.
One significant problem with NAND is that in order to overwrite a small amount of data, such as a 4KB block, an entire NAND cell must be updated, which may be 16KB in size. If no spare area is available, an even longer erase operation must be carried out before this cell can be written back. For example, four random 4KB blocks of written data could result in 64KB of NAND being overwritten. Not only does small write operations result in slower performance, but also significant write amplification where several times more physical storage space is overwritten than the amount of data being sent to the SSD.
With the ReRAM prototype, small writes are stored in ReRAM and only when enough data to fill a NAND block is written does this get transferred to NAND. Like volatile RAM, ReRAM can also be rewritten without a tedious erase cycle, so if data is overwritten before it gets transferred to NAND, this also saves another NAND write operation and the time involved. The prototype also uses an algorithm to store frequently accessed data in the ReRAM due to the much reduced latency of reading from ReRAM compared to NAND.
Based on testing their prototype controller with an emulator, they found that write performance improved by 11 times, power consumption reduced by 93% and the estimated NAND life increased by 6.9%. As most write operations generally involve small amounts of data being written, they also estimate that data ends up being written in the ReRAM 30 times more frequently than in the NAND flash memory.
So far it is unclear how long they are away from bringing this to the market, let alone the premium over NAND-only SSDs, but it appears to be targeted at the enterprise market, especially database servers that constantly write small amounts of data.
Tech-On has a detailed technical article including diagrams of this prototype.