docs(mysql-table-cache): document why Table_open_cache_overflows is not tracked

Closes #968

Author: markuslf

CHANGELOG.md

@@ -95,6 +95,7 @@ Monitoring Plugins:
 * file-ownership: `--filename` now merges with the default file list instead of replacing it; use `--no-default-files` to check only user-supplied files
 * file-ownership: extend default file list with CIS benchmark-relevant files (login.defs, sudoers, sysctl, systemd, PAM, etc.)
 * file-ownership: use `os.stat()` instead of shelling out to `stat`, improving performance and robustness
+* mysql-table-cache: document in the plugin README why `Table_open_cache_overflows` is intentionally not tracked (it is routine cache housekeeping, not a pass/fail signal) and link to the MariaDB KB for tuning guidance; the check logic itself is unchanged and remains in sync with MySQLTuner v2.8.38 ([#968](https://github.com/Linuxfabrik/monitoring-plugins/issues/968))
 * nextcloud-version: modernize code
 * php-status: always assume http://localhost/monitoring.php and, if not found, be tolerant
 * redis-status, valkey-status: modernize code and unify both plugins again after [PR #954](https://github.com/Linuxfabrik/monitoring-plugins/pull/954)

check-plugins/mysql-table-cache/README.md

@@ -3,12 +3,13 @@
 
 ## Overview
 
-Checks the hit rate for open table cache lookups in MySQL/MariaDB. A low hit rate indicates that `table_open_cache` may need to be increased. Logic is taken from [MySQLTuner script](https://github.com/major/MySQLTuner-perl):mysql_stats(), v1.9.8.
+Checks the hit rate for open table cache lookups in MySQL/MariaDB. A low hit rate indicates that `table_open_cache` may need to be increased. Logic is taken from [MySQLTuner](https://github.com/jmrenouard/MySQLTuner-perl):mysql_stats() and has been verified in sync with MySQLTuner v2.8.38.
 
 **Important Notes:**
 
 * Requires MySQL/MariaDB v5.1+
 * See [additional notes for all mysql monitoring plugins](https://linuxfabrik.github.io/monitoring-plugins/plugins-mysql/)
+* `Table_open_cache_overflows` is intentionally not tracked. The MySQL reference manual describes it as "the number of times, after a table is opened or closed, a cache instance has an unused entry and the size of the instance is larger than `table_open_cache / table_open_cache_instances`" ([MySQL Server Status Variables](https://docs.oracle.com/cd/E17952_01/mysql-5.7-en/server-status-variables.html)). In other words, it is a routine cache-housekeeping counter that increments whenever MySQL temporarily extends a per-instance cache bucket above its allocated share, so a non-zero value is not by itself a problem. A [MySQL 5.6 benchmark by Dimitri Kravtchuk](https://planet.mysql.com/entry/?id=34237) shows healthy servers running with non-zero overflows for hours. The existing `table_cache_hit_rate < 20%` warn threshold already covers the "table_open_cache is too small" signal and matches the same heuristic MySQLTuner v2.8.38 still uses. See [MariaDB KB: Optimizing table_open_cache](https://mariadb.com/kb/en/library/optimizing-table_open_cache/) for further tuning guidance.
 
 **Data Collection:**
 
@@ -98,4 +99,4 @@ Output:
 * License: The Unlicense, see [LICENSE file](https://unlicense.org/).
 * Credits:
 
-    * heavily inspired by MySQLTuner (<https://github.com/major/MySQLTuner-perl>)
+    * heavily inspired by MySQLTuner (<https://github.com/jmrenouard/MySQLTuner-perl>)

check-plugins/mysql-table-cache/mysql-table-cache

@@ -20,7 +20,7 @@ import lib.human
 from lib.globals import STATE_OK, STATE_UNKNOWN, STATE_WARN
 
 __author__ = 'Linuxfabrik GmbH, Zurich/Switzerland'
-__version__ = '2026040801'
+__version__ = '2026041401'
 
 DESCRIPTION = """Checks the hit rate for open table cache lookups in MySQL/MariaDB. A low hit rate
 indicates that table_open_cache may need to be increased.
@@ -109,8 +109,13 @@ def get_status(conn):
 def main():
     """The main function. This is where the magic happens."""
 
-    # logic taken from mysqltuner.pl:mysql_stats(), section # Table cache, v1.9.8
-    # including variable names
+    # logic taken from mysqltuner.pl:mysql_stats(), section # Table cache,
+    # verified in sync with MySQLTuner v2.8.38 (the hit-rate formula, the
+    # `Opened_tables > 0` calc guard, the `Open_tables > 0` check guard,
+    # the `< 20%` warn threshold and the "Increase table_open_cache
+    # gradually" / "open_files_limit must be greater than
+    # table_open_cache" recommendation are unchanged upstream since the
+    # original port).
 
     # parse the command line, exit with UNKNOWN if it fails
     try: