1. A method of managing a data storage device including a memory controller and a memory device, the method comprising: for a write request received at the data storage device: calculating a first sequential and consecutive write cost (SCWC) according to a garbage collection (GC) write operation policy, calculating a second SCWC according to a slack space recycling (SSR) write operation policy, and calculating a third SCWC according to an in-place updating (IPU) write operation policy; determining a write operation policy which has a lowest cost among the first through third SCWCs; and writing data in a selected segment in the memory device according to the determined write operation policy.