Garbage Collection

Unlike hard drives or NOR flash, NAND flash does not have a fixed mapping of logical memory addresses to physical memory addresses. The assignment takes place via mapping tables, which are managed by the firmware of the storage medium.

The flash memory itself consists of several thousand blocks, and these in turn are made up of “pages”. A block is the smallest unit that can be deleted in one operation. A page is the smallest unit that can be programmed in one operation. This split means that outdated pages cannot easily be used for new data if there is still valid data in the same block. The clear up process of being able to release entire blocks is called “garbage collection”.