Third, the MRU (most recently used table) algorithm stores frequently accessed data in the ReRAM. It records a certain amount of the latest addresses that were used for writing data and sent by the host so that data having those addresses is written in the ReRAM. This is based on the assumption that the addresses of frequently accessed data is stored in the latest records.
When the research group prototyped a controller to carry out those controls and tested it on an emulator, it confirmed that SSD's data writing performance, power consumption and product life can be improved by 11 times, reduced by 93% and increased by 6.9 times, respectively. The group assumed that each of the controller, ReRAM and NAND flash memory were three-dimensionally connected by TSV (through silicon via). But is possible to achieve almost the same results without using TSV, it said.
As input data, the group used access data of financial servers, for which random access is dominant. And, to use the hybrid SSD architecture for different applications, it is necessary to change the algorithms, the group said.
Among the improvements announced this time, the about seven times longer life of NAND flash memory has the largest impact. At data centers, etc, SSDs have to be replaced periodically. Therefore, the longer life of NAND flash memory can reduce costs to 1/7. Even though ReRAM, which is currently expensive, is used, "It pays off enough," Takeuchi said.
Furthermore, the performance and reliability required for the ReRAM became clear. For the new hybrid SSD, the research group reduced the frequency of writing data in the NAND flash memory and decided to make heavy use of the ReRAM.
As a result, data is written in the ReRAM about 30 times more frequently than in the NAND flash memory. If data is rewritten in the NAND flash memory 3,000 times, data is rewritten in the ReRAM about 100,000 times (105 times). As for the reading and writing speeds required for the application, about several microseconds is enough.
Those values required for the ReRAM are much lower than expected. To use ReRAM as a storage-class memory such as for the new hybrid SSD, it is better to scale down manufacturing processes for increasing capacity and lowering costs than to develop technologies for improving reliability and speed, Takeuchi said.