Docs/io uring async#2
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Design doc for migrating the SSD data path from synchronous pread/pwrite (+ per-record fsync WAL) to an io_uring-based async engine for the LLM KVCache workload. Covers: three io_uring invariants, an IoUringEngine pinned to a brpc user_pthread (ring-per-io-thread, mixed read/write), the read path (batched SubmitReads + Index::BatchGet), the two write paths (Segment data->bitmap ordered via IO_LINK, WAL group commit), relaxed cache-semantics durability, O_DIRECT/4K alignment as a later phase, and a P1-P4 rollout. Co-Authored-By: Claude <noreply@anthropic.com>
Rewrite the write-path section: instead of two independent paths, a single KVCache write organizes slot data + bitmap + WAL into one io_uring submission unit chained data -> bitmap -> WAL(commit record) -> fsync via IO_LINK. WAL lands last (it holds the index pointer, not data, so data must be durable first). Adds full crash-recovery semantics (A/B/C crash points, orphan-slot handling strategies, the index->live-slot invariant) and folds durability into the merged model. Co-Authored-By: Claude <noreply@anthropic.com>
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