Whenever a file is opened for writing, a snapshot of the file is created. The snapshot includes a copy of the file's metadata. A snapshot file map is initialized, pointing nowhere. A bitmap of the blocks in the file map is initialized. As new data are written to a block in the file, the original contents of the block are copied to a duplicate block. The bitmap is updated to indicate that the contents of the block have changed. The snapshot file map is updated to point to the duplicate block, and the new data are written to the original block. To archive the file, the file system uses the bitmap to identify which blocks have changed since the file was opened. Unchanged blocks are read through the original file map, and changed blocks are read through the snapshot file map. Finally, when the file is closed and the archive operation is complete, the bitmap and the resources used by the snapshot are returned to the system.