Aofrewriteincrementalfsync

Aofrewriteincrementalfsync is a Redis configuration option that governs how the system persists the AOF (Append Only File) during the rewrite process. AOF persistence records every write operation, and periodically Redis rewrites the AOF to compact it. The incremental fsync option controls whether the rewritten AOF file is fsynced in small, periodic steps as it is built, rather than waiting until the rewrite is complete.

When incremental fsync is enabled, Redis performs the rewrite by generating a new temporary AOF file and, as this file grows, performing frequent, small fsync operations on portions of the file. This approach spreads disk I/O and durably commits more of the rewritten data earlier, which can help limit latency spikes associated with a full, single, long fsync at the end of the rewrite. If the option is disabled, Redis may delay or batch fsync operations until the rewrite finishes, potentially resulting in longer pauses during the rewrite but reduced per-step disk activity.

Configuration and usage: the behavior is controlled by the directive aof-rewrite-incremental-fsync, typically set to yes or no in the Redis configuration. The exact default value can depend on the Redis version and platform, so users should verify their current setup. In practice, enabling incremental fsync is often beneficial for latency-sensitive deployments, but it can increase disk I/O during the rewrite.

Considerations: while incremental fsync can reduce rewrite-related pauses, it introduces more frequent disk synchronization work and may interact with the characteristics of the underlying filesystem or storage. Operators should test under representative workloads to determine the best setting for stability and performance.