MySQL Backup Failure Caused by Duplicate Cron Jobs and How to Fix It

Background: An online MySQL instance planned to upgrade to 8.0, and a test environment was built from a daily xtrabackup full backup that is compressed to about 300 GB; decompression repeatedly fails with gzip errors.

Fault analysis: The backup script, executed by crontab, streams xtrabackup output to gzip and writes to xxx.tar.gz . Although the script reports success, the cron logs show that the same job is launched twice at the same minute, and two crond processes are running concurrently. Both processes write to the same archive, causing data overlap and corruption.

Root cause: The system had two crond daemons running, so each triggered the backup script simultaneously. After stopping one daemon and restarting crond , only a single process remained and the duplicate execution stopped.

Remediation: Add a file lock (e.g., flock ) to the script to ensure mutual exclusion, conduct regular backup‑restore drills, and monitor the number of crond processes, alerting when it differs from one.

Keywords: #backup failure #cron job.