Storage control apparatus, storage system, and virtual volume control method

The storage control apparatus of the present invention saves a table for managing a virtual volume in a pool and keeps the state of the table in the latest state. A first dynamic mapping table (DMT) that manages a first virtual volume is saved in a first pool. Upon receipt of a write command relating to an unused virtual slot from a write command issuing device, a first virtual volume control unit assigns an unused real slot in the first pool to the virtual slot and updates the first DMT. The first virtual volume control unit discriminates the validity of the received data and, in cases where "0" data are received, releases the assigned real slot, updates the first DMT once again, and discards the received data. In cases where the received data are valid data, de-staging is performed following a DMT update.