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On-disk refers to data that is stored physically on a disk medium, such as a hard disk drive or solid-state drive. It contrasts with data kept in volatile memory (RAM) that is lost when a program terminates or the system powers down. On-disk data persists beyond process lifetimes and requires input/output operations to retrieve.

In computing, on-disk storage is managed by file systems and databases. File systems organize files, directories, and metadata on disk, while databases store tables, indexes, and transaction logs on disk and use in-memory caches and buffer pools to speed access. Data can be accessed directly via disk blocks or through abstractions such as memory-mapped I/O, which maps disk pages into a process’s address space.

The use of on-disk storage involves trade-offs. It offers high capacity and durability, but access is typically slower than in-memory operations. Performance is enhanced through caching, prefetching, sequential reads, indexing, and buffering. Reliability features such as journaling, checksums, and redundancy help protect data against corruption or hardware failure.

Security and disaster recovery are important considerations for on-disk data. Encryption at rest protects stored data, and regular backups or replication (including RAID configurations) helps ensure availability and recoverability.

The term is often written as on-disk, on disk, or ondisk, with on-disk being common in technical writing. It is used across file systems, databases, and storage systems to distinguish persisted storage from in-memory data structures.