Slim Git History

Slim Git History

Reclaim space and speed up git when a repo's .git has grown to multiple GB (the digitalmodel #1142 pattern). Works per-repo; repeat across repos.

Safety rules (READ FIRST — these are non-negotiable)

• The agent NEVER force-pushes — it is auto-denied, and history rewrite needs one. The agent does analysis + planning; the operator runs the rewrite + push.
• NEVER strip a currently-tracked path without explicit per-file confirmation. Stripping removes it from the working tree entirely — committed deliverables (client reports, datasets) are often deliberately version-controlled. The TRACKED-vs-GONE classification in step 1 exists to prevent this.
• Always back up before any rewrite (mirror clone or git bundle).
• A blob-strip must leave HEAD's tree byte-identical. It only removes already-deleted (GONE) blobs, so the current tree cannot change. Verify this with the content-parity gate (analyze-repo-bloat.sh --verify-parity, step 6) BEFORE the force-push — a non-zero exit means the rewrite dropped a live deliverable. This is the guard that would have caught the 2026-06 slim silently dropping digitalmodel #989 (the DNV-OS-F101 module) from main.
• The non-rewrite wins (step 2) are always safe — do them regardless, even if the rewrite is deferred.

Step 1 — Measure & classify (read-only)

bash scripts/analyze-repo-bloat.sh [repo-path] [top-N]

Reports .git size, pack/loose-object counts, and the largest blob paths in history, each tagged TRACKED (still in current tree — keep) or GONE (already deleted — safe to strip). The reclaim estimate is the sum of GONE bytes. If .git is small or all big blobs are TRACKED, stop — a rewrite isn't warranted.

Step 1.5 — Reachable or unreachable? (decides the whole approach)

Before planning a rewrite, find out whether the bloat is reachable history or just unreachable cruft — they need completely different fixes:

# reachable bytes (what a fresh mirror clone would keep) vs on-disk store:
git rev-list --disk-usage --objects --all      # reachable bytes
du -sb "$(git rev-parse --path-format=absolute --git-common-dir)"   # on-disk bytes

• reachable ≈ on-disk → bloat lives in reachable history → go to step 3 (filter-repo + operator force-push). [most repos]
• reachable ≪ on-disk (e.g. 0.2 GB reachable, 2.4 GB on disk) → the rest is unreachable cruft (classic on a shared repo whose cron auto-sync does repeated git reset --hard origin/main). Reclaim it with NO history rewrite and NO force-push — pause auto-sync, then:

  git reflog expire --expire=now --all && git gc --prune=now
  

  Plain git gc (Step 2) only prunes cruft older than 2 weeks, so active churn needs --prune=now + the reflog expiry. Then apply step 3 only to any reachable remainder. The analyzer script prints this VERDICT automatically.

Step 2 — Safe non-rewrite wins (no force-push, do always)

git gc                                # 14 packs -> 1-2, prune loose objects
git config core.untrackedCache true   # stop git status re-stat'ing the whole tree
git update-index --untracked-cache

• Skip feature.manyFiles — it flips index.skipHash/index v4, which can break cron/auto-sync tooling on a shared store.
• gc consolidates and re-deltas (faster object access) but does not shrink reclaimable reachable history — that needs step 3 (or, for unreachable cruft, the --prune=now in step 1.5).
• Run git gc only when no auto-sync / concurrent git op is mid-flight.

Step 3 — Plan the rewrite (target only GONE blobs)

Pick the narrowest filter that hits the GONE bytes. Common forms:

# by size (strips every blob over the threshold, from all history):
git filter-repo --strip-blobs-bigger-than 10M --analyze   # dry-run report first
git filter-repo --strip-blobs-bigger-than 10M

# by path (preferred when GONE bytes are concentrated in known dirs):
git filter-repo --path path/to/old-junk/ --invert-paths

Build an explicit keep-list from the TRACKED rows. Prefer --path over a blanket size strip when a size threshold would also catch a TRACKED deliverable.

Step 4 — Operator executes (STOP and hand off)

The agent stops here and hands the operator this sequence:

git clone --mirror <repo> ../<repo>-backup.git     # 1. backup
git filter-repo <planned args>                      # 2. rewrite (in a fresh clone)
# 3. CONTENT-PARITY GATE — must print PASS (exit 0) before pushing (#989 guard):
bash <workspace-hub>/.claude/skills/devops/slim-git-history/scripts/analyze-repo-bloat.sh \
     --verify-parity ../<repo>-backup.git .
git push --force --all && git push --force --tags    # 4. force-push (OPERATOR) — only after PASS
git gc --prune=now --aggressive                      # 5. reclaim locally

If step 3 prints FAIL, the rewrite dropped a still-tracked file — stop, do not push, and narrow the filter (a --strip-blobs-bigger-than threshold that also caught a TRACKED deliverable is the usual cause; switch to --path).

Step 5 — Coordination (shared-repo hazards)

• Pause the cron auto-sync first — it does git reset --hard origin/main and will fight the rewrite or clobber it.
• After force-push, every clone, worktree, and CI runner must re-clone or hard reset to the rewritten history.
• Open PRs are invalidated (their commit SHAs no longer exist) and must be recreated from the rewritten base.

Step 6 — Verify

# CONTENT PARITY (the load-bearing check — run BEFORE force-push, gates it):
analyze-repo-bloat.sh --verify-parity ../<repo>-backup.git .   # must print PASS
du -sh .git              # confirm shrink
git fsck --full          # integrity
git clone <remote> /tmp/smoke && cd /tmp/smoke && git log --oneline -1   # fresh-clone smoke test

Why parity is first: du/fsck/smoke-test confirm the repo is smaller and internally intact — but a rewrite that silently dropped a merged deliverable passes all three (it's smaller, it fsck's clean, it clones). Only the parity gate compares the live HEAD tree against the pre-rewrite backup and fails when a tracked file vanished. A GONE-blob strip must leave the tree byte-identical; any diff is a dropped deliverable (the digitalmodel #989 failure mode).

See scripts/analyze-repo-bloat.sh for the analysis details.