sentences:  To reduce storage requirements, if an imaging utility is filesystem-aware it can omit copying unused space, and it can compress the used space.Disk images were originally (in the late 1960s) used for backup and disk cloning of mainframe disk media.The early ones were as small as 5 megabytes and as large as 330 megabytes, and the copy medium was magnetic tape, which ran as large as 200 megabytes per reel.