twofile

Twofile is a data organization pattern used in computing and digital storage whereby a dataset is represented by two related files: a primary data file containing the raw or serialized content, and a secondary file that holds metadata, structure information, or an index describing the data. The two files are designed to be kept in sync, and the index or metadata enables selective access and streamlined updates without modifying the entire data file.

Use cases include archival storage for large binary objects, scientific datasets, and multimedia archives, where separating data from descriptive information can improve manageability and access efficiency. There is no universal standard for how two-file storage is implemented, and different systems may place metadata in different formats or locations.

Design considerations include naming conventions, alignment between data and index, data integrity checks, and recovery procedures in case of corruption. Access patterns typically begin with the index to locate data blocks, followed by reading the corresponding data segments. Benefits can include modular metadata management and faster access to selective portions of the data, while drawbacks include the risk of desynchronization and increased complexity.

Common variants pair a data file with an accompanying index or metadata file, such as data.bin paired with data.idx or data.meta. Some deployments use a header or manifest file to describe the structure, whereas others embed less information in the data file and rely on the separate descriptor.

See also data management, metadata, indexing, and file formats.