Efficient Read Disturb Management Schemes in Resource-Constrained Flash Memory Controller

Whenever a read operation is performed in a flash memory storage device, the surrounding cells in the same block are affected and their reliability gradually decreases. This phenomenon is known as read disturb problem. A remedy to this read disturb is to count the number of read operations for each block and perform read reclaim, the process of moving the existing data to a new block when the count reaches a predefined threshold. However, as the number of blocks in the flash memory device increases, maintaining the per-block read count requires a great amount of memory space in the controller. Therefore, system designers are forced to maintain read counts for a group of blocks (i.e. superblock) which results in the loss of accuracy. This paper proposes novel read disturb management schemes that can reduce the number of read reclaims significantly even if the read count is maintained for each superblock. In the proposed Pointer-based scheme, we keep track of the last block read so as to avoid excessive increase in the read count when the data is read sequentially. We also propose the Bitmap-based scheme that can successfully approximate the actual read count in the presence of random reads, with a negligible space overhead. Our experiments with real-world traces show that the Pointer-based scheme and the Bitmap-based scheme reduce the number of read reclaims by 65.5% and 90.5% on average, respectively, compared to the conventional scheme.