STORAGE DEVICE, STORAGE CONTROL METHOD, AND COMPUTER-READABLE RECORDING MEDIUM STORING STORAGE CONTROL PROGRAM

A storage device comprising a storage including a first pool area providing a physical area in units of first chunks of first size, and a second pool area providing a physical area in units of second chunks of second size smaller than the first size, and a controller. The controller counts a first count value indicating a number of first divided areas in which data is stored from among a plurality of the first divided areas obtained by dividing, by the second size, a virtual volume that has the first chunk allocated only to an area in which data is stored. In a case where a ratio of the first count value to a number of the first chunks allocated to the virtual volume is equal to or less than a first threshold, allocate the second chunk instead of the first chunk to the virtual volume.