1. A method of operating at least one data processor for maintaining a production dataset in computer data storage, and during concurrent read-write access to the production dataset, also maintaining a snapshot copy of the production dataset, the snapshot copy being the state of the production dataset at a point in time when the snapshot copy is created, the production dataset including blocks of computer data, and the snapshot copy of the production dataset also including blocks of computer data, wherein the method comprises: responding to a request for writing new data to a block of the production dataset by saving original data of the block of the production dataset, and writing the new data to the block of the production dataset, and keeping a record that the block has been changed in the production dataset since the point in time when the snapshot copy was created, wherein the record that the block has been changed in the production dataset since the point in time when the snapshot copy was created is kept by writing the new data or the original data to a sparse file so that blocks of the production dataset that have been changed since the point in time when the snapshot copy was created are indicated by a block allocation map of the sparse file.