Rebuild: the merger stops when a pass over the file keys decides nothing

Waiting files counted as work, so the merge loop re-read them for its
whole 30 s budget every minute: Postgres used ~a core even without
workers. Now ~4% at rest, and storing a boneless range costs Postgres
~0.7–1.9 s of CPU (it was 33 s with the loop running). Chunk payloads
are stored EXTERNAL (zstd already; no pglz attempt).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

app/services/fragment_merger.ts

@@ -12,6 +12,8 @@ export interface MergeResult {
   completed: number
   incomplete: number
   waiting: number
+  /** This round reached the end of the file keys (the next one starts over). */
+  wrapped: boolean
 }
 
 /** File keys per round. */
@@ -29,14 +31,15 @@ class FragmentMerger {
   private cursor = ''
 
   async run(): Promise<MergeResult> {
-    const result: MergeResult = { files: 0, completed: 0, incomplete: 0, waiting: 0 }
+    const result: MergeResult = { files: 0, completed: 0, incomplete: 0, waiting: 0, wrapped: false }
     const { rows: keys } = await db.rawQuery(
       `SELECT DISTINCT f.file_key FROM fragments f JOIN ranges r ON r.id = f.range_id
        WHERE r.status = 'done' AND f.file_key > ? ORDER BY f.file_key LIMIT ?`,
       [this.cursor, BATCH]
     )
     // At the end of the keys: the next round starts over.
-    this.cursor = keys.length < BATCH ? '' : keys.at(-1).file_key
+    result.wrapped = keys.length < BATCH
+    this.cursor = result.wrapped ? '' : keys.at(-1).file_key
     if (keys.length === 0) return result
     const rules = new RuleSet(await RuleSets.current())
     const rows = await db

commands/coordinator_run.ts

@@ -48,14 +48,20 @@ export default class CoordinatorRun extends BaseCommand {
         }
         if (Date.now() - mergedAt >= MERGE_EVERY_MS) {
           mergedAt = Date.now()
-          // Rounds until nothing more can be decided, or MERGE_BUDGET_MS.
+          // Rounds until a whole pass over the file keys decides nothing more, or MERGE_BUDGET_MS.
+          // (Going on while only waiting files were left kept Postgres busy all the time.)
           const totals = { completed: 0, incomplete: 0, waiting: 0 }
+          let decidedThisPass = 0
           while (!stopping && Date.now() - mergedAt < MERGE_BUDGET_MS) {
             const merged = await FragmentMerger.run()
             totals.completed += merged.completed
             totals.incomplete += merged.incomplete
             totals.waiting = merged.waiting
-            if (merged.files === 0) break
+            decidedThisPass += merged.completed + merged.incomplete
+            if (merged.wrapped) {
+              if (decidedThisPass === 0) break
+              decidedThisPass = 0
+            }
           }
           if (totals.completed || totals.incomplete) this.logger.info(`Fragments: ${totals.completed} files completed, ${totals.incomplete} incomplete, ${totals.waiting} waiting.`)
         }

database/migrations/1770000000500_create_release_chunks.ts

@@ -24,6 +24,8 @@ export default class extends BaseSchema {
         created_at timestamptz NOT NULL DEFAULT now(),
         PRIMARY KEY (id, created_at)
       ) PARTITION BY RANGE (created_at)${where}`)
+    // The payload is zstd already: no pglz attempt on it.
+    this.schema.raw(`ALTER TABLE release_chunks ALTER COLUMN payload SET STORAGE EXTERNAL`)
     this.schema.raw(`CREATE INDEX release_chunks_release_idx ON release_chunks (release_id)`)
     this.schema.raw(`CREATE INDEX release_chunks_range_idx ON release_chunks (range_id)`)
   }