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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>
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