> For the complete documentation index, see [llms.txt](https://polarischain-1.gitbook.io/polarischain/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://polarischain-1.gitbook.io/polarischain/7.-storage-and-persistence-architecture/7.1-write-ahead-log-wal-and-durability-guarantees.md).

# 7.1 - Write-Ahead Log (WAL) & Durability Guarantees

The WAL serves as the foundational durability layer, ensuring all consensus operations are persisted before state transitions are applied. Designed for high-throughput sequential I/O, the WAL implements:

* **Memory-Mapped Append:** 16 MB `mmap` chunks enable zero-copy sequential writes, minimizing syscall overhead and maximizing disk throughput.
* **Tagged Entry Format:** Each record is prefixed with a type-identifying tag byte and explicit length field, enabling precise parsing and backward-compatible extension.
* **Entry-Aligned Truncation:** A sidecar index tracks exact record boundaries. Size-based pruning never truncates mid-record, preserving crash consistency.
* **Reference-Counted Lifecycle:** A lightweight wrapper tracks long-lived WAL references, surfacing potential memory leaks via telemetry before they trigger out-of-memory conditions.
* **Deterministic Replay:** On recovery, the WAL is replayed from the last known checkpoint to reconstruct in-memory consensus state without data loss.


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