1. A method of operating a memory system including a memory controller and a nonvolatile memory device, the method comprising: storing data in a buffer region of the nonvolatile memory device under the control of the memory controller; by operation of the memory controller, issuing a migration request directed to the data stored in the buffer region; executing a migration operation in the nonvolatile memory device in response to the migration request, where the migration operation moves the data from the buffer region to a main region of the nonvolatile memory device; upon completion of the migration operation, marking a migration operation completion time, and after an initial verify shift (IVS) time has elapsed following the migration operation completion time, updating a mapping table associated with the data in view of the executed migration operation; receiving a read request from a host; and performing a read operation to retrieve the data from one of the buffer region in response to the read request, wherein if the read request is received before the initial verify shift (IVS) time has elapsed following the migration operation completion time, the data is retrieved from the buffer region, and if the read request is received after the initial verify shift (IVS) time has elapsed following the migration operation completion time, the data is retrieved from the main region, wherein the nonvolatile memory device includes at least one three dimensional memory block, the at least one three dimensional memory block including a plurality of strings connected between a bit line and a common source line, each of the plurality of strings including a plurality of memory cells connected in a series, each of the plurality of memory cells being connected to word lines stacked in a direction perpendicular to a substrate.