SET GLOBAL max_allowed_packet doesn't work

问题

I found out how to change the default value of max_allowed_packet in MySQL using SET GLOBAL. However, each time I used this command, the default value stayed untouched! I used these commands:

mysql --user=root --password=mypass
mysql> SET GLOBAL max_allowed_packet=32*1024*1024;
Query OK, 0 rows affected (0.00 secs)
mysql> SHOW VARIABLES max_allowed_packet;

And then the result is max_allowed_packet = 1048576. What am I missing?

回答1:

Hmmmm.. You have hit this NOT-A-BUG it seems. :)

If you change a global system variable, the value is remembered and used for new connections until the server restarts. (To make a global system variable setting permanent, you should set it in an option file.) The change is visible to any client that accesses that global variable. However, the change affects the corresponding session variable only for clients that connect after the change. The global variable change does not affect the session variable for any client that is currently connected (not even that of the client that issues the SET GLOBAL statement).

Refer this too. Read Shane Bester explanation.

You should change from the my.ini/my.cnf file and restart the server for the max_allowed_packet setting to take effect.

回答2:

After running

set global max_allowed_packet=1000000000;

you have to restart mysql before

SHOW VARIABLES LIKE 'max_allowed_packet'

will show the new value.

I have this issue when restarting mysql through the MAC OSX system preferences and the value hadn't changed. So by logging into mysql via console

mysql -u root -p

changing it and then restarting mySql seemed to work. Might have been a OS X quirk though.

回答3:

Just a quick way to see the variable for anybody who comes across this. To get the value back you need to run

SHOW VARIABLES LIKE 'max_allowed_packet'

回答4:

I came across this problem as well and in my case I have multiple versions of MySql installed. Adding this note for anyone who might have setup MySql using homebrew on mac and are having trouble setting max_allowed_packet value in your my.cnf file.

The most key information that helped was that the my.cnf file can be present in different locations (excerpt from https://github.com/rajivkanaujia/alphaworks/wiki/Install-MySQL-using-Homebrew) -

/usr/local/etc/my.cnf  -->  Global Access
/usr/local/etc/my.cnf  -->  Global Access
/usr/local/Cellar/mysql/5.7.18/my.cnf   --> Sever Level Access
~/.my.cnf   --> User Level Access

Since I installed MySql 5.6 via Home brew I found it at -

/usr/local/Cellar/mysql\@5.6/5.6.43/my.cnf

Steps followed -

• Update the /usr/local/Cellar/mysql\@5.6/5.6.43/my.cnf file under [mysqld] group with the necessary max_allowed_packet value -

  [mysqld] max_allowed_packet=5G

• Restart mysql using brew services -

  brew services restart mysql@5.6

• Connect/Reconnect to the mysql shell and verify that the configuration has taken effect using -

  show variables like 'max_allowed_packet';

回答5:

For those with a MariaDb configuration the problem could be that the max_allowed_packet variable is overwritten by a configuration file called later.

In my case I tried to import a database and the server answered me: ERROR 2006 (HY000) at line 736: MySQL server has gone away

I discovered that the file:

/etc/mysql/mariadb.conf.d/50-server.cnf

is called later

/etc/mysql/conf.d/mysql.cnf

I tried continuously changing in the "mysql.cnf" file but the value was overwritten in "50-server.cnf".

So the solution is to enter the file

/etc/mysql/mariadb.conf.d/50-server.cnf

and instead of "max_allowed_packet = 16M" put the desired value as an example "max_allowed_packet = 64M"

来源：https://stackoverflow.com/questions/9922894/set-global-max-allowed-packet-doesnt-work