Memory system controlling nonvolatile memory

A controller configured to control a nonvolatile memory, the nonvolatile memory including a plurality of first storage regions and a second storage region, each of the plurality of first storage regions and the second storage region including a memory cell, the controller comprising:

• an interface configured to be connected with a host;
• and
• a circuit configured to:
• obtain, from a write buffer of the host, first write data associated with one or more first write requests, the one or more first write requests being for writing data to a first write destination storage region allocated from the plurality of first storage regions;
• write the first write data to the first write destination storage region in a first write mode for writing m-bit data per memory cell, m being an integer greater than or equal to two;
• obtain, from the write buffer of the host, second write data associated with one or more second write requests, the one or more second write requests being for writing data to a second write destination storage region allocated from the plurality of first storage regions;
• write the second write data to the second write destination storage region in the first write mode;
• and
• in causing a state of one of the plurality of first storage regions to transition from a first state of being allocated as a write destination storage region in which writing of data is possible to a second state in which writing of data is suspended,
• obtain, from the write buffer of the host, third write data which are associated with the one or more first write requests and have not been obtained;
• write the third write data to the second storage region shared by the plurality of first storage regions, in a second write mode for writing n-bit data per memory cell, n being an integer greater than or equal to one and less than or equal to m;
• and
• cause the state of the one of the plurality of first storage regions to transition from the first state to the second state.