According to one embodiment, a memory device includes a nonvolatile memory which stores data in units of a write unit includes cells, and a controller which controls the memory and partitions memory space of the memory. In response to a request to write write-data to the memory from a host, the controller requests the host to transmit a segment of the write-data with a specified size. The write-data segment has a size of an integral multiple of a size determined to allow for a set of the write-data segment and corresponding additional data to be the largest while smaller than the write unit. Before completion of processing a first command which requests access to a first partition, the controller accepts a second command which requests access to a second partition.