docs: close D3 fc/OpenZFS upstream review (VALIDATED, no code change)

[HardcoreMonk]

요약

백로그 "fc upstream/OpenZFS 참고 보고 검토(D3)"를 VALIDATED로 종결한다(docs-only, 코드 변경 없음). anvil의 D3 방어(coarse-hole 파일시스템에서 sparse diff snapshot 오염 방지)를 upstream Firecracker diff-snapshot + OpenZFS hole-보고 의미론에 대조해 검증했고, 전 지점 정합·fail-safe·근본적 내구임을 확인했다.

D3 배경

ZFS(recordsize>4K)는 sparse diff의 hole을 record 단위로만 보고 → SEEK_HOLE이 written 영역을 과대보고 → 미기록 padding이 base memory를 덮어써 guest triple-fault(D3). anvil 방어(PR #36): ProbeHoleGranularity로 fs granularity 측정 → 창설측 diff→full 강등(applyD3DiffGuard) + 판독측 overlay 거부 + recordsize=4K dataset 권장.

검증 (upstream 대조, deep-worker 조사 + 컨트롤러 코드 검증)

Q	결과	핵심 근거
fc diff = 4K sparse dirty-page? HoleGranularityFine=4096 정확?	✅	fc snapshot-support.md(sparse dirty diff), KVM 4K dirty-tracking. fc 자체 merge 도구도 동일 SEEK_HOLE overlay → 취약성은 fc 포맷 고유
ZFS SEEK_HOLE = recordsize 정렬?	✅	FreeBSD D36194(hole 단위=recordsize), dnode_next_offset block 탐색
SEEK_HOLE가 미-sync dirty 미보고 → fsync+retry 옳은 가드?	✅	zfs_dmu_offset_next_sync man
방어 sound·complete·fail-safe?	✅	lseek(2) 계약이 loophole, probe는 모든 불확실=coarse, 엣지(recordsize==4K/<4K/≥256K/ENOTSUP/거짓fs) 전부 안전
완화 가능?	근본적·내구	recordsize 과대보고는 ZFS 할당 모델 고유(어떤 버전도 finer 미보고). 유일 완화 = 이미 권장하는 recordsize=4K

컨트롤러 검증(코드 대조): review의 핵심 Note 1(read-side 가드는 현재 fs를 probe, 태생 fs 아님 → 창설측 강등의 보편성에 의존)을 직접 확인 — snapshot 생성 경로는 단일(api.go resolveSnapshotType→applyD3DiffGuard), coarse fs에서 항상 diff→full 강등하므로 anvil은 coarse-born diff를 절대 생성하지 않음 → Note 1은 이론적 residual(legacy/non-anvil diff)일 뿐 실무 gap 아님.

변경

• docs/operations/2026-07-19-d3-upstream-review.md 신설 — durable 검토 요약(Q1–Q5, 소스, Note 1·2, fail-safe·엣지, 결론).
• CONTEXT.md — D3 리뷰 백로그 라인 종결 + (겸사) refactor(web): migrate operator UI to Svelte 5 runes (core, behavior-preserving) #87 병합됐으나 미종결이던 runes 마이그레이션 백로그 라인도 종결(doc-sync).

부차 발견 (검토 밖·선택, 비차단)

• dead code: storage.HoleGranularityCoarse 헬퍼는 production 호출자 0(데몬은 ProbeHoleGranularity 직접) — 선택적 cleanup 후보(별도 작업).
• (선택) fc 자체 rebase-snap도 동일 취약 → fc 문서 노트 제보 후보(anvil 책임 아님).

lifecycle

docs 검토·종결(코드/계약 변경 없음). deep-worker(Opus) upstream 조사 + 컨트롤러 코드 검증. gofmt clean(.go 무변경).

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• CONTEXT.md
• docs/operations/2026-07-19-d3-upstream-review.md

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