Methods, systems, and apparatus, including computer programs encoded on computer storage media, for writing data in one or more storage units. One of the methods includes obtaining an erasing count for each of multiple storage units, wherein the erasing count equals a total count of erasing operations that have been performed on all blocks of the storage unit. The method further includes identifying one or more of the storage units that satisfy one or more conditions associated with writing data, determining a storage unit that has the smallest erasing count among the identified storage units, and writing the data in the determined storage unit.