The term Write Once Read Many (WORM) refers to storage media or systems that allow data to be recorded a single time and thereafter only read. WORM preserves the original bit pattern so that subsequent modification, overwriting, or deletion is prevented by design or by policy enforcement.

WORM implementations fall into two broad categories: physical WORM (media that is literally irreversible, such as CD-R or DVD-R) and logical or emulated WORM (systems that enforce immutability at the device, filesystem, or application level). Examples include optical write-once discs, certain tape cartridges configured for write-once use, and disk or cloud appliances offering immutable object storage.

Characteristics

  • Single-write guarantee: data can be written only once and then read many times without alteration.
  • Read durability: designed for long-term readability rather than frequent random writes.
  • Tamper resistance: prevents accidental or intentional alteration, aiding integrity and legal defensibility.
  • Enforcement mechanisms: may rely on physical media properties, firmware controls, or software/policy locks.

Historically, WORM was adopted for legal and regulatory retention of financial, medical, and corporate records. Consumer and enterprise optical media made write-once storage widely available; later, specialized tape formats, appliance-based WORM drives, and object-storage features extended capacity and manageability.

Common uses today include secure logging, immutable backups, compliance archives, and forensic retention. Modern storage vendors and cloud services often provide object locking or retention policies that emulate WORM behavior while offering indexable, searchable repositories. For more technical classification and standards, see related storage resources.

Limitations include the inability to amend data, potentially higher long-term management overhead, and the need to plan capacity and retrieval strategy. Distinguishing true physical write-once media from software-enforced immutability helps organizations choose the appropriate WORM approach for archival integrity, compliance, and operational needs.