Memory system and method of controlling memory system

A memory system comprising:

• a non-volatile semiconductor memory having a plurality of first storage areas, the first storage areas being capable of including one or more second storage areas, a plurality of third storage areas in which data is written in a first mode, and a plurality of fourth storage areas in which data is written in a second mode, the number of bits that can be written to one storage area in the second mode being greater than that in the first mode;
• and
• processing circuitry configured to:
• acquire a destination storage area from the one or more second storage areas;
• compare total number of the third storage areas and a first threshold value to determine whether or not the total number of the third storage areas is greater than the first threshold value;
• when the total number of the third storage areas is greater than the first threshold value, compare total rate of valid data of the third storage areas and a second threshold value to determine whether or not the total rate of valid data of the third storage areas is smaller than the second threshold value;
• select a source storage area from the plurality of fourth storage areas when the total number of the third storage areas is greater than the first threshold value and when the total rate of valid data of the third storage areas is smaller than the second threshold value;
• select a source storage area from the plurality of third storage areas when the total number of the third storage areas is equal to or smaller than the first threshold value or when the total rate of valid data of the third storage areas is equal to or greater than the second threshold value;
• and
• move the valid data stored in the selected source storage area into the acquired destination storage area using the second mode.