WriteBehind

Write-behind, also known as delayed write, is a data writing strategy used to improve performance by acknowledging write requests before the data is permanently stored. In this approach, write operations are first written to a fast cache and only later flushed to the slower backing store, such as a disk or database. The actual persistence occurs asynchronously, allowing higher throughput and lower perceived latency for write requests.

The mechanism typically relies on a write-back cache that holds recently written data and related metadata. A flush policy determines when cached data is written to storage. Triggers can include a timer, cache eviction, explicit flush commands, or system shutdown events. Some implementations use battery-backed or non-volatile memory to preserve cached data in case of power loss, reducing the risk of data loss. Writes may also be coalesced to optimize I/O by combining multiple pending writes into fewer disk operations.

Advantages of write-behind include improved write throughput, reduced disk seeks, and better batching of I/O, which can lower latency for individual writes in the short term. The primary risk is data loss or inconsistency if the system crashes or loses power before the cached data is flushed. Recovery requires mechanisms such as durable cache hardware or persistent journaling to reconstruct or ensure data integrity after an outage.

Common contexts for write-behind include storage controllers, RAID adapters, and operating systems’ page or device caches. It is not the same as write-through caching, nor is it equivalent to write-ahead logging, which is used to ensure durability in databases.