Memory system including a memory device that can determine optimum read voltage applied to a word line

A memory system comprising:

• a nonvolatile memory including a word line and a plurality of memory cells connected to the word line;
• and
• a controller configured to:
• perform a first threshold voltage tracking operation, during which the controller determines an optimum read voltage for the plurality of memory cells connected to the word line based on data read from the plurality of memory cells using multiple read voltages;
• calculate an exhaustion degree of the plurality of memory cells connected to the word line based on the number of times of erasing of a block including the plurality of memory cells connected to the word line;
• and
• when the calculated exhaustion degree is less than a predetermined threshold, perform a second threshold voltage tracking operation to cause the nonvolatile memory to determine an optimum read voltage, wherein, during the second threshold voltage tracking operation, the controller transmits to the nonvolatile memory, a command that causes the nonvolatile memory to search for an optimum read voltage for the plurality of memory cells connected to the word line, wherein
• the nonvolatile memory is configured to, in response to the command, search an optimum read voltage for the plurality of memory cells connected to the word line using multiple read voltages, and to transmit a response including a search result.