Patent ID: 7577788

Claim:
A disk array system connected with a plurality of host computers via a network, comprising: a disk control apparatus; and a plurality of hard disk drives configured into at least a first logical volume and a second logical volume, wherein the disk control apparatus stores a plurality of sets of update data, each set of which includes at least one byte of data and is written at a respective update time into the first logical volume in response to a respective update request sent from one of said host computers, whenever a difference period between said respective update time of one set of the plurality of sets of update data and a current time point exceeds a length of a predetermined time period, the disk control apparatus relocates said one set of update data stored in the first logical volume and stores said one set of relocated update data only into the second logical volume, said respective update time of the one set of relocated update data is a prior time point earlier than the current time point, the difference period between the prior time point and the current time point is equal to or longer than the length of the predetermined time period, and only some sets of the plurality of sets of update data, which are stored in the first logical volume in response to a plurality of said update requests sent during the predetermined time period, remain in the first logical volume as of the current time point, and the disk control apparatus provides a virtual volume in response to receiving a split order at a split time point, and maps the virtual volume to both the second logical volume stored only with a plurality of said one set of relocated update data as are at the split time point and the first logical volume stored only with a plurality of said one set of update data written thereinto as are at the split time point, wherein the split time point is equal to or earlier than the current time point.