Memory device and host device

A memory device comprising:

• a nonvolatile semiconductor memory having a plurality of physical storage areas that include an externally accessible user area, the user area being divided into a plurality of allocation units, the plurality of allocation units being divided into recording units;
• and
• a control unit configured to control the nonvolatile semiconductor memory, wherein the control unit is configured to:
• receive one or more control commands to designate a sequential write area for writing data;
• assign, as the sequential write area, an allocation unit, of the plurality of allocation units, represented by an address of the one or more control commands;
• receive one or more write commands, wherein:
• the one or more write commands indicate access to an area, in the sequential write area, that has not been written;
• a data length, indicated by the one or more write commands, is equivalent to one or more recording units;
• and
• a data write location, indicated by the one or more write commands, is on a boundary of a recording unit;
• and
• write data, associated with the one or more write commands, at an average speed that satisfies a write performance index.