Filebacked

Filebacked is a term used to describe data or memory that relies on a file on the filesystem as its backing store. In this sense, the data is not held solely in volatile memory but is persisted to disk, either for durability, size, or sharing between processes. The concept is common in systems that balance performance with persistence, such as databases, caches, and large data processing tasks.

A common mechanism for filebacked memory is memory-mapped I/O. A region of a file is mapped into a process’s address space, so reads and writes to the mapped region access the file directly. The operating system caches pages in memory and may load or evict them as needed. Writes to the mapped area can be propagated to disk immediately or deferred until the system flushes the pages. This approach enables handling data larger than available RAM and allows multiple processes to share the same data through shared mappings.

Filebacked structures are often used for persistence with low-level control over when data is written to disk. Examples include databases that store large indexes or segments in files, and libraries that provide on-disk maps or queues. Memory-mapped files are also used for inter-process communication and for implementing large, pageable datasets without loading everything into RAM at once.

Trade-offs include higher latency compared with pure in-memory access, potential complexity in ensuring data durability and consistency, and dependence on filesystem performance and reliability. Proper synchronization, flushing, and error handling are essential to avoid data loss.