With hard disks, there is a fixed assignment between logical and physical addresses. With NAND flash, however, existing data cannot simply be overwritten because the area must be erased first. For technological reasons, erasing can only be done on entire flash blocks, which are typically several megabytes in size. Therefore, data that the host writes to the same logical block address is stored at a different position in the NAND flash each time. The memory position is thereby stored in a table also in the NAND, allowing quick access to the data during a read.
To manage the data in the NAND efficiently, manufacturers established a common size of 4 KiB. This is the smallest management unit used by today’s file systems (so-called cluster). It is important that the file system clusters are aligned with the management boundaries of the NAND flash. Otherwise, delays will occur, especially with random write or read accesses.