Performance and Data Safety
When Fluent Bit processes data, it uses the system memory (heap) as a primary and temporary place to store the record logs before they get delivered, in this private memory area the records are processed.
Buffering refers to the ability to store the records somewhere, and while they are processed and delivered, still be able to store more. Buffering in memory is the fastest mechanism, but there are certain scenarios where it requires special strategies to deal with backpressure, data safety or reduce memory consumption by the service in constrained environments.
Fluent Bit as buffering strategies go, offers a primary buffering mechanism in memory and an optional secondary one using the file system. With this hybrid solution you can accommodate any use case safely and keep a high performance while processing your data.
Both mechanisms are not mutually exclusive and when the data is ready to be processed or delivered it will always be in memory, while other data in the queue might be in the file system until is ready to be processed and moved up to memory.