write once, read many

A WORM (write once, read many) device is optical or magnetic storage media where data cannot be overwritten or erased once committed to the surface. The recording layer physically changes state during writing, usually through a laser burning marks into a polycarbonate substrate or a phase-change material. Once that layer is altered, it stays altered. Users can read the data as many times as needed, but no subsequent write operation can touch those sectors.

WORM media comes in several physical formats. Optical discs (CD-R, DVD-R) use a dye layer that darkens irreversibly when struck by a laser at high power; reading uses lower laser power that does not alter the marks. Magneto-optical discs employ a different principle: a laser heats material above its Curie point while a magnetic field sets polarity, creating marks that cannot be rewritten. Some early phase-change media operated under WORM constraints before rewritable versions became standard.

WORM storage is mandated in regulated industries where data integrity and audit trails are legal requirements. Financial institutions, healthcare facilities, and pharmaceutical manufacturers use WORM to guarantee that transaction records, patient files, and batch documentation cannot be altered retroactively. A regulator or auditor can trust that a file with a 2015 timestamp was actually written in 2015 and has not been tampered with since. Backup archives and cold storage also benefit from the write-protection guarantee.

The practical limitation is capacity and speed compared to rewritable media. A WORM DVD holds 4.7 GB, similar to a standard DVD-R, but once filled it becomes read-only permanently. Organizations must manage multiple WORM discs as archives grow. Jukebox systems with robotic changers can handle hundreds of discs, selecting and loading media automatically for retrieval. However, this approach is now legacy; cloud and tape-based archival with cryptographic verification have largely displaced WORM optical media in new installations.

The term reflects the asymmetry of the operation: information flows in one direction only. Write happens once per sector; read happens many times. This simplicity is its strength in compliance contexts. A WORM disc cannot be accidentally overwritten during routine storage or retrieval, and no firmware bug or user error can corrupt archived data. The trade-off is convenience and flexibility, which is why WORM is reserved for archive and compliance use cases rather than general-purpose storage.