Expand metrics checkpoint profiling

docs/release-control/v6/internal/records/known-rc-issue-closure-for-ga-metrics-write-amplification-2026-05-03.md

@@ -92,6 +92,17 @@ production 4,000-page auto-checkpoint enabled, a traced 157,452-sample run
 reduced total process write bytes from 392,445,952 to 257,208,320 and `fsync`
 calls from 42 to 32.
 
+Checkpoint threshold sweeps did not justify changing the production
+4,000-page setting. With no concurrent reader, 8k, 16k, and 32k thresholds
+reduced the 30-tick total from 1,501,548,544 bytes to 1,399,660,544,
+1,142,358,016, and 1,018,478,592 bytes, but grew the observed WAL allocation
+from 40,112,352 bytes to 51,306,392, 89,745,992, and 155,525,912 bytes. More
+importantly, with a paced reader on the production four-connection pool the
+16k threshold wrote 1,134,166,016 bytes versus 1,047,703,552 at 4k and retained
+a larger WAL allocation. The benefit was therefore not robust under the
+concurrent-read path that #1601 made release-critical, so the 16 MB nominal
+checkpoint bound remains unchanged.
+
 The migration keeps the legacy unique index authoritative until deferred
 startup maintenance transactionally swaps the indexes. Tests cover legacy
 duplicates, close/restart, a process exit between index drop and replacement,

docs/release-control/v6/internal/subsystems/performance-and-scalability.md

@@ -1217,7 +1217,10 @@ host exposes them, and traced sync calls rather than treating final file size
 as a proxy. Its checked invariant persists 157,452 samples across 2,197 mixed
 provider resources and caps the no-auto-checkpoint workload at 40,000 WAL
 frames; the former four-index schema produces 50,516 frames while the
-consolidated schema produces 35,030.
+consolidated schema produces 35,030. Checkpoint-threshold changes must also
+prove physical writes, WAL allocation, write latency, restart, and the
+four-connection concurrent-read case; a no-reader byte reduction alone is not
+sufficient to widen the production WAL bound.
 Retention must also return freed SQLite pages to the OS
 proportionally to the current freelist, bounded per cycle, and on every
 retention cycle rather than only when that cycle deleted rows: a fixed small