Storage device that secures a block for a stream or namespace and system having the storage device

A memory system comprising:

• a nonvolatile semiconductor memory device including a plurality of physical blocks;
• and
• a memory controller configured to:
• manage free blocks which are to be allocated for writing data;
• receive from a host a first command including a first ID and first information, the first information designating a first amount of physical blocks;
• allocate the designated first amount of physical blocks for the first ID;
• receive from the host a second command including a second ID and second information, the second information designating a second amount of physical blocks;
• allocate the designated second amount of physical blocks for the second ID;
• receive from the host a third command including the first ID and an instruction to write first data;
• in response to receiving the third command, write the first data into one of the physical blocks among the allocated first amount of physical blocks;
• receive from the host a fourth command including the second ID and an instruction to write second data;
• and
• in response to receiving the fourth command, write the second data into one of the physical blocks among the allocated second amount of physical blocks.