The core of the architecture is a very high performance and very low latency b+ tree, which also is not a conventional b+ tree but more of hybrid, taking concepts from compacting data structures: btree nodes are very large, log structured, and compacted (resorted) as necessary in memory. This means our b+ trees are very shallow compared to other filesystems.

What this means for the end user is that since we require very few seeks or disk reads, filesystem latency is extremely good - especially cache cold filesystem latency, which does not show up in most benchmarks but has a huge impact on real world performance, as well as how fast the system “feels” in normal interactive usage. Latency has been a major focus throughout the codebase - notably, we have assertions that we never hold b+ tree locks while doing IO, and the btree transaction layer makes it easily to aggressively drop and retake locks as needed - one major goal of bcachefs is to be the first general purpose soft realtime filesystem.

Additionally, unlike other COW btrees, btree updates are journalled. This greatly improves our write efficiency on random update workloads, as it means btree writes are only done when we have a large block of updates, or when required by memory reclaim or journal reclaim.