dbaas/mysql: innodb_doublewrite=DETECT_ONLY to halve page-flush writes

MySQL device-write investigation (code-oflt): after the nextcloud webcal
throttle settled (the earlier 3.4-8.8 MB/s were post-restart transients),
MySQL is ~1.74 MB/s at the InnoDB level — and HALF of that (~0.86 MB/s,
~55 pages/s) is the doublewrite buffer writing every flushed page twice.
Redo is negligible (0.01 MB/s), no temp-table spilling.

Set innodb_doublewrite=DETECT_ONLY (dynamic, no restart; persisted in the
cnf): InnoDB stops writing full page CONTENT to the doublewrite buffer
(~halves MySQL's page-flush writes on the IOPS-bound sdc) but keeps
torn-page DETECTION metadata — a crash-torn page is flagged on recovery
(restore from the daily mysqldump) rather than silently corrupt. Chosen
over full OFF: same write saving, keeps detection, and OFF requires a
shutdown ("cannot change to OFF if doublewrite is enabled"). Acceptable
risk given the PERC BBU cache + UPS (in-flight writes complete on power
loss) + daily per-db backups.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

stacks/dbaas/modules/dbaas/main.tf

@@ -112,6 +112,11 @@ resource "kubernetes_config_map" "mysql_standalone_cnf" {
       innodb_io_capacity_max=200
       innodb_redo_log_capacity=1073741824
       innodb_buffer_pool_size=2147483648
+      # DETECT_ONLY: stop writing full page content to the doublewrite buffer
+      # (~halves page-flush writes on the IOPS-bound sdc) while keeping torn-page
+      # DETECTION; recovery is restore-from-backup. OK with BBU+UPS+daily
+      # mysqldump. Dynamic (no restart). code-oflt 2026-06-30.
+      innodb_doublewrite=DETECT_ONLY
       innodb_flush_neighbors=1
       innodb_lru_scan_depth=256
       innodb_page_cleaners=1