perf(media): the apply fsyncs the journal once per batch, not per move

The 58k-rename apply on the spinning array spent most of its wall time in
fsyncs: one durability flush per rename, and a quadratic `Journal.already()`
scan over a growing record list on top of it. Both are the journal's, not the
renames'.

- `Journal` indexes move intents by (src, dst) so `already()` is O(1).
- `--batch N` (default 64) writes N intents, fsyncs once, then performs those
  N renames. The invariant is unchanged: no rename runs before its own intent
  is durable, so a crash mid-batch replays the batch and resumes.
- Tests: a mid-batch kill loses nothing and the rerun completes; batching
  changes no final path and no journal line.

scripts/media-apply.py

@@ -59,10 +59,18 @@ def journal_default(root):
 
 
 class Journal:
-    """Append-only JSONL of intents. Writes are fsynced before the tool acts."""
+    """Append-only JSONL of intents. Writes are fsynced before the tool acts.
 
-    def __init__(self, path, writing):
+    ``batch`` is how many intents may be written before one fsync covers them.
+    The rule the file exists for is unchanged — an intent is on disk before the
+    move it authorises runs — so the caller must call ``sync()`` before it
+    performs a batch's renames; only the number of flushes falls. 1 = a flush
+    per intent (the original behaviour).
+    """
+
+    def __init__(self, path, writing, batch=1):
         self.path, self.writing = path, writing
+        self.batch = max(1, int(batch))
         self.records = []
         if os.path.exists(path):
             for line in open(path):
@@ -70,11 +78,15 @@ class Journal:
                 if line:
                     self.records.append(json.loads(line))
         self._fh = None
+        self._unsynced = 0
+        # (src, dst) index: already() is called three times per move, so an
+        # any() over every record written so far made a 58k-move pass quadratic.
+        self._intents = {(r.get("src"), r.get("dst"))
+                         for r in self.records if r.get("op") == "move"}
 
     def already(self, dst, src):
         """An intent for this exact (src, dst) was written by an earlier run."""
-        return any(r.get("op") == "move" and r.get("dst") == dst
+        return (src, dst) in self._intents
-                   and r.get("src") == src for r in self.records)
 
     @property
     def rolled_back(self):
@@ -82,16 +94,28 @@ class Journal:
 
     def record(self, **fields):
         self.records.append(fields)
+        if fields.get("op") == "move":
+            self._intents.add((fields.get("src"), fields.get("dst")))
         if not self.writing:
             return
         if self._fh is None:
             self._fh = open(self.path, "a")
         self._fh.write(json.dumps(fields) + "\n")
         self._fh.flush()
+        self._unsynced += 1
+        if self._unsynced >= self.batch:
+            self.sync()
+
+    def sync(self):
+        """Make every intent written so far durable. No-op when nothing pending."""
+        if not self.writing or self._fh is None or not self._unsynced:
+            return
         os.fsync(self._fh.fileno())
+        self._unsynced = 0
 
     def close(self):
         if self._fh:
+            self.sync()
             self._fh.close()
 
 
@@ -177,14 +201,30 @@ def do_apply(args, root, journal, mkdirs, keeps, moves):
             print(f"  would move {src} -> {dst}")
         return 0
 
+    batch = max(1, args.batch)
     made = {r.get("dst") for r in journal.records if r.get("op") == "mkdir"}
     dirs_new = 0
+    queued = []
+
+    def drain_dirs():
+        """Create the queued folders only after their intents are durable."""
+        nonlocal dirs_new
+        if not queued:
+            return
+        journal.sync()
+        for d in queued:
+            os.makedirs(d, exist_ok=True)
+            dirs_new += 1
+        queued.clear()
+
     for d in sorted(mkdirs, key=lambda q: q.count(os.sep)):
         if d in made or os.path.isdir(d):
             continue
         journal.record(op="mkdir", dst=d)
-        os.makedirs(d, exist_ok=True)
+        queued.append(d)
-        dirs_new += 1
+        if len(queued) >= batch:
+            drain_dirs()
+    drain_dirs()
 
     # same-filesystem gate: rename cannot cross a device, and a silent copy of a
     # library this size is not an option anyone would choose.
@@ -201,6 +241,19 @@ def do_apply(args, root, journal, mkdirs, keeps, moves):
         return 2
 
     moved = done = 0
+    pending = []
+
+    def drain_moves():
+        """Perform the queued renames only after their intents are on disk."""
+        nonlocal moved
+        if not pending:
+            return
+        journal.sync()
+        for s, d in pending:
+            os.rename(s, d)
+            moved += 1
+        pending.clear()
+
     for src, dst, _kind in moves:
         if journal.already(dst, src):
             if os.path.exists(dst) and not os.path.exists(src):
@@ -213,8 +266,10 @@ def do_apply(args, root, journal, mkdirs, keeps, moves):
         st = os.stat(src)
         journal.record(op="move", src=src, dst=dst, bytes=st.st_size, mode=st.st_mode,
                        mtime=st.st_mtime, ino=st.st_ino, dev=st.st_dev)
-        os.rename(src, dst)
+        pending.append((src, dst))
-        moved += 1
+        if len(pending) >= batch:
+            drain_moves()
+    drain_moves()
     print(f"applied: {moved} moved, {done} already done in an earlier run,"
           f" {dirs_new} folders created, {len(keeps)} files stayed put")
     print(f"journal: {journal.path} — undo with: media-apply.py rollback"
@@ -418,13 +473,17 @@ def main():
     ap.add_argument("--metadata-view", default=METADATA_VIEW,
                     help="overlay the media servers read through; the sidecar"
                          " note prints only when this path is a mountpoint")
+    ap.add_argument("--batch", type=int, default=64,
+                    help="journal intents per fsync (1 = flush before every move)."
+                         " An intent is still durable before the move it authorises;"
+                         " only the number of flushes falls")
     args = ap.parse_args()
 
     root = os.path.abspath(args.root)
     if not os.path.isdir(root):
         raise Refuse(f"no library at {root}")
     journal_path = args.journal or journal_default(root)
-    journal = Journal(journal_path, args.apply)
+    journal = Journal(journal_path, args.apply, batch=args.batch)
     try:
         if args.cmd in ("rollback", "verify", "sweep") and not os.path.exists(journal_path):
             raise Refuse(f"no journal at {journal_path} — nothing to {args.cmd}")

serverAppConfigs-docs/Dankserver/Media Relayout.md

@@ -81,6 +81,13 @@ An apply also leaves the release folders it emptied standing where they were —
 hundreds of them at the library root — which is what `sweep` removes, dry-run
 first, from the same journal.
 
+`--batch N` (default 64) makes the journal durable per batch instead of per
+move: N intents are written and fsynced, then those N rename. The rule a crash
+relies on — no rename runs before its intent is on disk — is unchanged, so an
+interrupted apply still replays the batch and resumes with nothing lost. Batch 1
+is the old behaviour; the only reason to name it is debugging, because on the
+spinning array it spends most of a large apply's wall time in fsyncs.
+
 After it lands, both servers need a scan or the change is invisible. [[Plex]] and
 [[Jellyfin]] own the exact requests; two laws hold at this step: `204` is
 acceptance and anything else queued nothing — re-issue the identical request

tests/media_relayout.bats

@@ -494,6 +494,65 @@ for r in moves:
 PY
 }
 
+# The live pass paid an fsync per move on a spinning array (63 074 of them) and
+# the run took 11 minutes for 58 278 renames. --batch N writes N intents, syncs
+# once, then performs those N moves — the durability rule is unchanged (no move
+# runs before its intent is on disk), only the number of flushes falls.
+@test "a batched apply journals every move and lands the same tree" {
+    plan
+    JOURNAL="$BATS_TEST_TMPDIR/batched.jsonl"
+    run python3 "$APPLY" apply --plan "$OUT" --root "$LIB" --journal "$JOURNAL" \
+        --batch 4 --apply
+    [ "$status" -eq 0 ]
+    local planned journalled
+    planned=$(awk -F'\t' '$1=="move"' "$OUT" | wc -l)
+    journalled=$(grep -c '"op": "move"' "$JOURNAL")
+    [ "$journalled" -eq "$planned" ]
+    run python3 "$APPLY" verify --plan "$OUT" --root "$LIB" --journal "$JOURNAL"
+    [ "$status" -eq 0 ]
+    python3 - "$OUT" <<'PY'
+import os, sys
+moves = [r for r in (l.rstrip("\n").split("\t") for l in open(sys.argv[1]))
+         if r[0] == "move"]
+assert moves, "fixture planned no moves"
+for r in moves:
+    assert os.path.exists(r[2]), f"target missing: {r[2]}"
+    assert not os.path.exists(r[1]), f"source left behind: {r[1]}"
+PY
+}
+
+@test "an apply stopped mid-batch resumes without losing or double-journalling a move" {
+    plan
+    JOURNAL="$BATS_TEST_TMPDIR/interrupted-batch.jsonl"
+    python3 "$APPLY" apply --plan "$OUT" --root "$LIB" --journal "$JOURNAL" \
+        --batch 2 --apply > "$BATS_TEST_TMPDIR/first.log" 2>&1 &
+    local pid=$! i
+    # stop the run the moment a batch of intents is on disk (it may also have
+    # finished already — both outcomes must resume cleanly)
+    for i in $(seq 1 200); do
+        [ -f "$JOURNAL" ] && [ "$(grep -c '"op": "move"' "$JOURNAL" 2>/dev/null || true)" -ge 2 ] && break
+        sleep 0.02
+    done
+    kill -9 "$pid" 2>/dev/null || true
+    wait "$pid" 2>/dev/null || true
+
+    run python3 "$APPLY" apply --plan "$OUT" --root "$LIB" --journal "$JOURNAL" --batch 2 --apply
+    [ "$status" -eq 0 ]
+    run python3 "$APPLY" verify --plan "$OUT" --root "$LIB" --journal "$JOURNAL"
+    [ "$status" -eq 0 ]
+    python3 - "$OUT" "$JOURNAL" <<'PY'
+import json, os, sys
+moves = [r for r in (l.rstrip("\n").split("\t") for l in open(sys.argv[1]))
+         if r[0] == "move"]
+journal = [json.loads(l) for l in open(sys.argv[2])]
+dsts = [j["dst"] for j in journal if j.get("op") == "move"]
+assert len(dsts) == len(set(dsts)), "a destination was journalled twice"
+assert len(dsts) == len(moves), f"journalled {len(dsts)} of {len(moves)} moves"
+for r in moves:
+    assert os.path.exists(r[2]) and not os.path.exists(r[1]), r[2]
+PY
+}
+
 @test "rollback puts every file back where the census found it" {
     plan
     apply_plan