Memory system and method of controlling nonvolatile memory

A memory system comprising:

• a nonvolatile memory including a plurality of memory cells, each of the plurality of memory cells being configured to store, in a nonvolatile manner, a plurality of bits of data including at least a first bit and a second bit, the plurality of memory cells including at least a first memory cell and a second memory cell different from the first memory cell;
• and
• a memory controller electrically coupled to the nonvolatile memory and configured to:
• in a case where the first memory cell stores valid first bit data as the first bit and does not store data as the second bit, and the second memory cell stores valid second bit data as the first bit and does not store data as the second bit, and upon reception of a flush command from a host,
• read the first bit data from the first memory cell;
• write the first bit data to the first memory cell as the second bit;
• read the second bit data from the second memory cell;
• and
• write the second bit data to the second memory cell as the second bit.

A method of controlling a nonvolatile memory, the nonvolatile memory including a plurality of memory cells, each of the plurality of memory cells being configured to store, in a nonvolatile manner, a plurality of bits of data including at least a first bit and a second bit, the plurality of memory cells including at least a first memory cell and a second memory cell different from the first memory cell, the method comprising:

• determining that the first memory cell stores valid first bit data as the first bit and does not store data as the second bit, and the second memory cell stores valid second bit data as the first bit and does not store data as the second bit;
• receiving a flush command from a host;
• and
• upon the reception of the flush command from the host,
• reading the second bit data from the second memory cell;
• and
• writing the second bit data read from the second memory cell to the first memory cell as the second bit.