Magnetic disk device and method for executing write command

A magnetic disk device comprising:

• a disk;
• a nonvolatile cache;
• a controller;
• and
• a backup power supply configured to temporarily generate power supplied at least to the nonvolatile cache and the controller when a main power supply to the magnetic disk device is interrupted,
• wherein the controller is configured to:
• start to receive first data of a data length specified in a first write command when receiving the first write command from a host;
• start to write the first data to the disk in accordance with the first write command;
• and
• report a status for execution of the first write command to the host depending on whether or not a second capacity of data not yet written to the disk is less than or equal to a first capacity of a first free space in the nonvolatile cache, the first free space being available to save data during a first period when the backup power supply enables power to be supplied, the second capacity decreasing as writing of the first data to the disk progresses.