perf(durable-streams-rust): per-shard checkpoint triggers — interval knob + retained-WAL size budget#4704
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balegas merged 3 commits intoJul 13, 2026
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…knob + retained-WAL size budget Replace the hardcoded 3s all-shards checkpoint wave with a per-shard dual trigger: --wal-checkpoint-interval-ms (time, default 3000 = old behavior) and --wal-checkpoint-wal-bytes (size, 0 = off): checkpoint a shard as soon as its retained WAL exceeds the budget. Rationale (wal-decomp-lane0, 2026-07-13): acks never gate on checkpoint and reads never touch the WAL, so checkpoint frequency is purely a replay-time budget — a size trigger makes that budget explicit and lets shards self-stagger by their own write rates instead of storming the device together. Each shard's clock restarts when its checkpoint finishes, an in-flight guard prevents re-firing a slow shard, and due shards still checkpoint concurrently. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01Y3x7bcT9vLGiT4tXZeQpnk
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…-path configuration Co-Authored-By: Claude Fable 5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01Y3x7bcT9vLGiT4tXZeQpnk
…ed; note the separate ~1M-stream wall Co-Authored-By: Claude Fable 5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01Y3x7bcT9vLGiT4tXZeQpnk
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Stacked validation complete ( Extreme-cardinality check on the same config: 244k @500k, 56k @1m — a separate, new mechanism appears near 1M streams (fd-per-stream vs nofile ceiling / 1M-file directory / map working set — not the checkpoint storm this stack fixed). Tracked as the next investigation. |
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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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Stacked on #4697. Replaces the hardcoded 3s all-shards checkpoint wave with a per-shard dual trigger:
--wal-checkpoint-interval-ms— time trigger, default 3000 (= old behavior).--wal-checkpoint-wal-bytes— size trigger (0 = off): checkpoint a shard as soon as ITS retained WAL exceeds the budget.Why
The checkpoint's only job is bounding retained-WAL size = crash-replay time: acks never gate on it and reads never touch the WAL (reads are sendfile from the data file's page cache, which is written before the WAL ack barrier). So cadence is purely a replay-time budget — the size trigger makes that budget explicit, and shards self-stagger by their own write rates instead of storming the device in a synchronized wave. Each shard's clock restarts when it finishes; an in-flight guard prevents re-firing a slow shard; due shards still checkpoint concurrently.
Results (c4d-standard-64-lssd, split-lane layout,
--wal-shards 5 --wal-checkpoint-syncfs on; suitewal-sizetrigger)--wal-checkpoint-wal-bytes 1GiB, 60s fallback)The size trigger recovers the entire 7–11% checkpoint cost — throughput at the ckpt-off ceiling with replay bounded at ≤1 GiB retained WAL per shard.
Verification
🤖 Generated with Claude Code
https://claude.ai/code/session_01Y3x7bcT9vLGiT4tXZeQpnk