Selecting a snapshot method based on cache memory consumption

A storage apparatus storing data accessed by a host computer, the storage apparatus comprising:

• a first storing unit storing the data;
• a second storing unit storing snapshot images of the data stored by the first storing unit;
• a cache memory temporarily storing write data to be written into the first storing unit by the host computer before the write data is stored into the first storing unit;
• a controller writing the write data into the first storing unit;
• and
• a priority judging unit judging deletion priority of the snapshot images stored in the second storing unit for each of storage areas on the first storing unit corresponding to the snapshot images;
• wherein
• if an amount of consumption of the cache memory has not reached a predetermined threshold, the controller saves a first snapshot image of data stored in a write-destination storage area of the write data, into the second storing unit after storing the write data onto the cache memory and returning a completion response to the host computer, and then writes the write data stored by the cache memory into the first storing unit;
• if the amount of consumption of the cache memory has reached the pre-determined threshold, the controller saves the first snapshot image of the data stored in the write-destination storage area of the write data, into the second storing unit, and then returns the completion response to the host computer after writing the write data into the first storing unit;
• and
• in the case of saving the first snapshot image into the second storing unit after returning the completion response to the host computer, the controller performs the saving preferentially for a storage area with a low deletion priority on the first storing unit.