Qing Wu

Qing Wu

Developer ❤️ Functional Programming

Today while trying to test a database related behavior in CakePHP, I created a fixture which imports the 16k records from a existing table. After a painful long wait, MySQL gave a table full error.

So I’ve got a problem to solve. Connect to the test database, check the table size, which is 16M; but in development database, with InnoDB engine, that table is just 1.5M. Then I noticed the storage engine of the test db was Memory. After some Googling, I found the relevant section The MEMORY Storage Engine in MySQL documentation.

The maximum size of MEMORY tables is limited by the max_heap_table_size system variable, which has a default value of 16MB.

That makes a lot of sense. This option doesn’t yet exist in my /etc/mysql/my.cnf. Let’s add it to the bottom of the configuration file (I’ve made a mistake without realising it):

max_heap_table_size=128M

Restart MySQL server by running:

sudo service mysql restart

And then try again, and … still doesn’t work. Go back to MySQL documentation and look again at the section for option max_heap_table_size:

This variable is also used in conjunction with tmp_table_size to limit the size of internal in-memory tables.

OK, let’s add this option as well:

tmp_table_size=128M

And restart MySQL and try again, and … it still doesn’t work. After some more Googling, I found this answer on StackOverflow, which says

Add this to /etc/my.cnf

[mysqld]
tmp_table_size=2G
max_heap_table_size=2G

this will cover mysql restarts.

OK. So I’ve added the correct options, but to the wrong location. Those options must be added under the [mysqld] section in the configuration file. After moving them to the correct section in /etc/mysql/my.cnf and restarting MySQL server, my new test finally works as expected (but still very slow).

Retrospect

I realised a few things, after spending an hour tackling this issue:

  1. Setting up test fixture with existing records may not be the best way to do it.
  2. CakePHP uses memory storage engine for its test database on MySQL.
  3. The memory storage engine probably doesn’t have compression as InnoDB does, since it consumes a lot more space for the same table.
  4. In MySQL configuration file, different sections control different parts of MySQL, which is why adding the options to the bottom wouldn’t work.

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