MySQL Monitoring Guide 2026: Key Metrics, Slow Queries & Alerting

-- Check current connection usage vs max

SELECT
  @@max_connections AS max_connections,
  (SELECT COUNT(*) FROM information_schema.processlist) AS current_connections,
  ROUND(
    (SELECT COUNT(*) FROM information_schema.processlist) / @@max_connections * 100, 1
  ) AS pct_used;

-- Calculate buffer pool hit rate

SELECT
  (1 - Innodb_buffer_pool_reads / Innodb_buffer_pool_read_requests) * 100
    AS buffer_pool_hit_rate_pct
FROM (
  SELECT
    (SELECT VARIABLE_VALUE FROM performance_schema.global_status
     WHERE VARIABLE_NAME = 'Innodb_buffer_pool_reads') AS Innodb_buffer_pool_reads,
    (SELECT VARIABLE_VALUE FROM performance_schema.global_status
     WHERE VARIABLE_NAME = 'Innodb_buffer_pool_read_requests') AS Innodb_buffer_pool_read_requests
) t;

-- Enable slow query log (no restart needed)

SET GLOBAL slow_query_log = 'ON';
SET GLOBAL long_query_time = 1;         -- Log queries > 1 second
SET GLOBAL log_queries_not_using_indexes = 'ON';  -- Also log full table scans
SET GLOBAL slow_query_log_file = '/var/log/mysql/slow.log';

[mysqld]
slow_query_log = 1
slow_query_log_file = /var/log/mysql/slow.log
long_query_time = 1
log_queries_not_using_indexes = 1
log_slow_replica_statements = 1

-- Check replication status (run on replica)

-- MySQL 8.0+
SHOW REPLICA STATUSG

-- MySQL < 8.0
SHOW SLAVE STATUSG

-- Key fields to check:
-- Replica_IO_Running: Yes (must be Yes)
-- Replica_SQL_Running: Yes (must be Yes)
-- Seconds_Behind_Source: 0 (lag in seconds — alert > 30)
-- Last_SQL_Error: (empty = healthy)
-- Last_IO_Error: (empty = healthy)

groups:
  - name: mysql
    rules:
      - alert: MySQLDown
        expr: mysql_up == 0
        for: 0m
        labels:
          severity: critical
        annotations:
          summary: "MySQL instance {{ $labels.instance }} is down"

      - alert: MySQLTooManyConnections
        expr: max_over_time(mysql_global_status_threads_connected[1m]) / mysql_global_variables_max_connections > 0.8
        for: 2m
        labels:
          severity: warning
        annotations:
          summary: "MySQL connections at {{ $value | humanizePercentage }} of max"

      - alert: MySQLHighQPS
        expr: rate(mysql_global_status_questions[2m]) > 2 * avg_over_time(rate(mysql_global_status_questions[2m])[1h:5m])
        for: 5m
        labels:
          severity: warning
        annotations:
          summary: "MySQL QPS is 2x above baseline"

      - alert: MySQLSlowQueries
        expr: rate(mysql_global_status_slow_queries[5m]) > 0
        for: 2m
        labels:
          severity: warning
        annotations:
          summary: "MySQL slow queries detected: {{ $value }} per second"

      - alert: MySQLReplicationLag
        expr: mysql_slave_status_seconds_behind_master > 30
        for: 1m
        labels:
          severity: critical
        annotations:
          summary: "MySQL replication lag is {{ $value }}s on {{ $labels.instance }}"

      - alert: MySQLReplicationNotRunning
        expr: mysql_slave_status_slave_sql_running != 1 or mysql_slave_status_slave_io_running != 1
        for: 0m
        labels:
          severity: critical
        annotations:
          summary: "MySQL replication stopped on {{ $labels.instance }}"

      - alert: MySQLBufferPoolHitRate
        expr: (1 - rate(mysql_global_status_innodb_buffer_pool_reads[5m]) / rate(mysql_global_status_innodb_buffer_pool_read_requests[5m])) < 0.95
        for: 5m
        labels:
          severity: warning
        annotations:
          summary: "MySQL buffer pool hit rate is {{ $value | humanizePercentage }} (below 95%)"