set of all the snapshot images the snapshot administration unit 442 receives the data transmitted from the target volumes such as 410 420 and 430 in fig4 and determines whether to send the data to the snapshot volume management unit 446 to be stored after comparing the data with the snapshot images stored in the snapshot image storage unit 444 and changes the address of the stored data to the corresponding snapshot images in the snapshot image storage unit 444 please refer to fig5 a for each snapshot image stored in the snapshot image storage unit 444 in fig5 a the snapshot image 510 is divided into two parts a primary bitmap table 512 and a removed history data cow bitmap table 514 referred to as u201c cow bitmap table u201d thereinafter in each snapshot image 510 the primary bitmap table 512 includes a plurality of positions for storing storage spaces corresponding to the target volumes for example two positions of the primary bitmap table 512 are respectively used for storing a portion of the storage spaces of the target volumes 520 and 530 the positions of the storage spaces are mapped one to one to the target volumes each position of the cow bitmap table 514 is used for storing the corresponding position of the snapshot volume 540 the positions of the storage spaces are mapped one to one to that of the snapshot volume in a selected embodiment the sizes of the primary bitmap.