A snapshot copy facility responds to a request for writing new data to a block of a production dataset by saving original data of the block, and writing the new data to the block of the production dataset and keeping a record of the blocks of the production dataset have been changed since the point in time when the snapshot copy was created. For detection of corruption of the snapshot metadata and for portability of the snapshot copy software, a sparse file is used for keeping a record of the blocks of the production dataset that have been changed since the snapshot. For “copy on first write” snapshots, the sparse file is a “write once read many” (WORM) file to store old data of blocks. For “remap on write” snapshots, the sparse file is a “read-write” (RW) file to which writes to the production file are stored.