Patent ID: 6898681

Claim:
A system for providing a copy of data at a point in time, comprising: a data storage device, comprising a master store arranged to store blocks of data, a plurality of subsidiary stores each arranged to store point in time copy data comprising blocks of data copied from said master store at a particular point in time, and a plurality of bitmap stores each bitmap store being associated with a corresponding different one of the subsidiary stores and being configured to store data indicating when a data block of the master store differs from a corresponding data block stored in the associated subsidiary store; and a controller configured to generate point in time copies, the controller being arranged, in response to a request to generate a first point in time copy at a first point in time, to check, when a data block in the master store is about to be changed, whether the data in that data block at the first point in time has already been copied to a first one of the subsidiary stores, and, if not, to copy that data to the first subsidiary store, the controller further being arranged, in response to a request to generate a second point in time copy at a second point in time, to check, when a data block in the master store is about to be changed, whether the data in that data block at the second point in time has already been copied to a second one of the subsidiary stores, and, if not, to copy that data to the second subsidiary store, the controller also being arranged to maintain data indicating differences between the master store and each point in time copy by causing data to be stored in the associated bitmap store to indicate when a data block in the master store differs from a corresponding data block of the point in time copy.