refactor(durable-streams-rust): checkpoint cleanup — syncfs default-on (Linux), drop dead fsync fan-out#4707
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…n (Linux), drop dead fsync fan-out, refresh WAL_TUNING - --wal-checkpoint-syncfs defaults ON for Linux (validated +51% at the cliff, neutral below; off = escape hatch). Non-Linux unchanged (no syncfs). - Remove FSYNC_FANOUT / --wal-fsync-parallel: parallel per-file checkpoint fsync regressed in every controlled test (f8 -11% NVMe, f16 -19% constrained) and the syncfs barrier supersedes it. Flag accepted as a warning no-op so old deploy scripts don't crash. Checkpoint step 2 is now two modes: per-lane syncfs (Linux default) or the serial per-file fallback. - e2e resets restore the platform default instead of hard 'false'. - WAL_TUNING.md: stream-lanes results (1M wall broken), cardinality-based lane-split guidance, updated flag set, fd-ceiling note (#4706). Co-Authored-By: Claude Fable 5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01Y3x7bcT9vLGiT4tXZeQpnk
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July 13, 2026 21:54
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…yncfs checkpoints, per-shard triggers, stream lanes (#4697) Eliminates the WAL write cardinality cliff end-to-end. Formerly a 4-PR stack (#4704, #4705, #4707 folded in here). Full deployment guidance: `WAL_TUNING.md` (in this PR). **Validated by the canonical benchmark campaign 2026-07-14** (ds-bench#6: `results/REPORT.md` + `PROVENANCE.md`) run against this PR's head. ## The problem WAL-durable write throughput collapsed with stream count: **10.4k ops/s @100k streams** (vs ~110k @10k), degrading further under load. Two compounding causes: 1. The checkpoint issued **one `fdatasync` per touched stream** — an O(N) barrier storm sharing the device with commit fsyncs. 2. At extreme cardinality (~1M), checkpoint **writeback amplification** (~40× for small appends: dirty page + inode + extents + journal per file) saturated the single data device — one `syncfs` measured at 60–74 s. ## The changes | commit | what | measured effect | |---|---|---| | syncfs checkpoint | one `syncfs` barrier per stream lane instead of O(N-touched) `fdatasync` (unconditional on Linux) | +51% at the 100k cliff | | per-shard checkpoint triggers | `--wal-checkpoint-interval-ms` + `--wal-checkpoint-wal-bytes` (retained-WAL/replay budget; shards self-stagger, in-flight guard, panic-safe) | checkpoint cost ≈ 0 | | `--stream-lanes N` | hash stream data files across per-device lane dirs; N parallel per-lane syncfs; lane count persisted + validated on open | 1M wall broken: 68k → **212k** (syncfs 60–74s → 6–11s) | | cleanup | dead knobs removed (`--wal-fsync-parallel`, meta gates, meta-sweep toggles, `--tier local`); memory×tier rejected at startup; flag-clear WAL records decode as torn | smaller, safer surface | ## Results — canonical campaign 2026-07-14 (this PR's head, ideal configuration) **Write saturation** (`canonical-write`, 256 B appends, c4d-standard-64-lssd, 3 stream lanes + 3 WAL lanes, pinned cores): | streams | wal-ideal | memory (no-fsync ceiling) | |---|---|---| | 10k | 417k | 680k | | 100k | **382k (−8%, no cliff)** | 632k | vs 10.4k @100k pre-campaign = **37×**. At extreme cardinality (same config): 285k @500k, 212k @1m. **Reads** (unchanged by this PR, revalidated): catch-up 2.27–2.74 GiB/s, cardinality-flat 10→100 streams; SSE tail p99 2–3 ms up to 2048 connections. **Mixed interference**: 100k concurrent catch-up readers cost a pinned 50k ops/s write load nothing (303 MiB/s replay served alongside). The 2026-07-02 memory-mode delivery collapse no longer reproduces on this build: delivery tracks writes 1:1 to 127k/s (wal: keeps pace to ~40k, caps ~64k del/s at full write saturation). ## Correctness - The recoverability contract is unchanged: ack = WAL group-commit fdatasync; a WAL segment is recycled only after its records' stream bytes are fsynced AND the durable-tail map is persisted. All knobs only change *when/how* that sequence runs. - 105/105 tests: syncfs crash-recovery e2e, 3-lane crash-recovery + layout-spread + lane-mismatch rejection, memory-mode sidecar-skip regression, randomized crash sim. - CI green including all four conformance configs (`memory`, `wal-default`, `wal-read-offload-always`, `wal-resident-cache`). - Defaults preserve behavior except syncfs-on-Linux (extensively measured); `--stream-lanes` defaults to 1 = byte-identical layout. Changeset: patch. ## Follow-ups (tracked) - #4695 log-structured store (removes per-file writeback amplification structurally — the residual 100k→1M slope) - #4696 cold tier as durable store - #4706 lazy fd management (1 fd/live stream = 96% of the 1,048,576 ceiling at 1M streams) - #4709 io_uring experiment (measured as a regression in v1 form; default off, not for merge) 🤖 Generated with [Claude Code](https://claude.com/claude-code) https://claude.ai/code/session_01Y3x7bcT9vLGiT4tXZeQpnk --------- Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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Final PR of the cardinality-cliff stack (#4697 → #4704 → #4705 → this): now that the ideal disk-based implementation is validated end-to-end, clean up what the campaign obsoleted.
--wal-checkpoint-syncfsdefaults ON for Linux. Validated across the whole campaign: +51% at the 100k cliff, neutral-or-better everywhere below it, and with perf(durable-streams-rust): --stream-lanes — shard stream data files across per-device lane dirs #4705 it is onesyncfsper stream lane.offremains as the escape hatch; non-Linux is unaffected (nosyncfs, per-file loop as before).--wal-fsync-parallel/FSYNC_FANOUT. Parallel per-file checkpoint fsync regressed in every controlled test (f8 −11% on NVMe, f16 −19% CPU-constrained) — it steals device budget from the commit path — and the syncfs barrier supersedes it wholesale. The flag is accepted as a warning no-op so existing deploy scripts don't crash. Checkpoint durability (step 2) is now exactly two modes: per-lane syncfs (Linux default) or the serial per-file fallback.false.Verification
106/106 tests pass. No behavior change on non-Linux; on Linux the only default change is syncfs-on, measured extensively (suites: wal-syncfs, wal-splitlane, wal-sizetrigger, wal-stacked-1m, wal-streamlanes-1m).
🤖 Generated with Claude Code
https://claude.ai/code/session_01Y3x7bcT9vLGiT4tXZeQpnk