DESIGN AND PERFORMANCE EVALUATION OF FLASH MEMORY BACKUP SYSTEMS

Main memory databases are a promising approach to the realization of high-performance systems since they do not require low-speed secondary storages. One of the critical problems of such systems is the volatile nature of main memory. Hence, nonvolatile secondary storage cannot be removed for back-up and recovery. The authors have developed continuous back-up systems which realize low-overhead back-up procedures and quick recovery. A recent development has been flash memory, which can maintain its contents without electric power supply. It is expected that flash memory will realize high-speed nonvolatile storage. There are, however, some characteristics which are different from disk storages. For example, in flash memory, the block size of a sequential access is very small, e.g., 256 bytes. Thus we cannot use sequential access to retrieve large amounts of data, although optimization to utilize sequential access need not be considered. Although flash memory seems to be a good choice for secondary storages, the number of rewrites to a memory cell determines its lifetime. Current flash memory cannot be used as conventional secondary storage. Therefore, this paper discusses system architectures which utilize flash memory as back-up storage. Special consideration is given to reducing the number of rewrite operations. It is expected that in the proposed systems, flash memory storage can survive over 10 years. Performance evaluation is also discussed.