Methods and apparatus implementing a technique for retrieving requested data. In general, in one aspect, the technique includes receiving a request from an application to read a portion of an application file and consulting a filter-system file. The filter-system file contains processed data that corresponds to data in the application file and correspondence information indicating how the set of processed data corresponds to the data in the application file. The technique also includes deriving the requested portion of the application file from the processed data using the correspondence information, and delivering the requested portion of the application file to the application. In another aspect, the technique includes receiving data to be retrieved in response to a request from an application with reference to the application's view of the application file, processing the received data to generate processed data that corresponds to the received data, producing correspondence information indicating how the processed data corresponds to the received data is produced, and storing the processed data and the correspondence information in a filter-system file.