A method of managing a garbage collection (GC) operation on a flash memory includes: calculating an overall spare area in a flash memory, which includes a spare area in a plurality of spare blocks in the flash memory and at least two of a spare area in one or more target blocks corresponding to writing of user data based on host write commands, a spare area in one or more destination blocks corresponding to writing of valid data based on the GC operation and a spare area in a source block corresponding to reading of valid data based on the GC operation; determining an adjustment factor according to the overall spare area; and performing the GC operation on the source block according to a GC-to-host base ratio and the adjustment factor.