It doesn't work that way, the wear leveling is done on the level of the device (one of the advantages of it being an eMMC and not a plain NAND flash). It will shuffle blocks around reading and writing them in a manner that they all have a rougly same number of writes. The device doesn't even *know* about empty space (that's a filesystem level thing), it's all just blocks from it's perspective. Also note that wear leveling is just a part of the strategy, there are spare blocks, error correction algorithms, etc.