ERROR 2006 (HY000): MySQL server has gone away

What the message looks like

ERROR 2006 (HY000): MySQL server has gone away
PHP Warning: mysqli::query(): MySQL server has gone away in /var/www/app/db.php on line 42

Common causes

• The connection was idle longer than wait_timeout, the server closed it, and the app reused the dead connection, typical for long-running scripts and workers.
• A query or a row is larger than max_allowed_packet, for example a big import or a blob.
• The server crashed or was restarted, often killed by the OOM killer, so all connections are gone.
• A firewall, load balancer or proxy cuts idle connections earlier than the database does.
• The connection was ended with KILL or the server hit another limit.

Check

1. Idle timeout
   mysql -e "SHOW VARIABLES LIKE 'wait_timeout'"Copy
2. Maximum packet size
   mysql -e "SHOW VARIABLES LIKE 'max_allowed_packet'"Copy
3. Did the server restart? Uptime in seconds
   mysql -e "SHOW GLOBAL STATUS LIKE 'Uptime'"Copy
4. Crash, kill or restart in the log
   journalctl -u mariadb -u mysql --since '1 day ago' --no-pager | grep -iE 'killed|out of memory|ready for connections|shutdown' | tail -n 20Copy

Fix

• If Uptime is small or the log shows a kill, the server restarted. Find out why first (memory, crash). The error is only the consequence.
• For idle connections: let the app reconnect on this error, or check the connection before use (ping). Pools should recycle connections earlier than wait_timeout.
• For big queries and imports raise the limit on the server, for example max_allowed_packet = 64M in [mysqld], and on the client too (mysql --max-allowed-packet or mysqldump setting). The value is an example, size it to your data.
• Align the idle timeout of the firewall or proxy with wait_timeout, or send keepalives.
• Do not simply set wait_timeout very high. It piles up idle connections and leads to Too many connections.

FAQ

Why does it only happen after a long pause?

The server closed the idle connection after wait_timeout and the app tried to use it afterwards. The fix is on the client side: reconnect or recycle.

It happens only on big imports.

Then the packet is bigger than max_allowed_packet. Raise it on both server and client, or split the import into smaller statements.