Changing the tmp folder of mysql

This is answered in the documentation:

Where MySQL Stores Temporary Files

On Unix, MySQL uses the value of the TMPDIR environment variable as the path name of the directory in which to store temporary files. If TMPDIR is not set, MySQL uses the system default, which is usually /tmp, /var/tmp, or /usr/tmp.

On Windows, Netware and OS2, MySQL checks in order the values of the TMPDIR, TEMP, and TMP environment variables. For the first one found to be set, MySQL uses it and does not check those remaining. If none of TMPDIR, TEMP, or TMP are set, MySQL uses the Windows system default, which is usually C:\windows\temp.

Here is an example to move the mysqld tmpdir from /tmp to /run/mysqld which already exists on Ubuntu 13.04 and is a tmpfs (memory/ram):

sudo vim /etc/mysql/conf.d/local.cnf

Add:

[mysqld]
tmpdir = /run/mysqld

Then:

sudo service mysql restart

Verify:

SHOW VARIABLES LIKE 'tmpdir';

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If you get an error on MySQL restart, you may have AppArmor enabled:

sudo vim /etc/apparmor.d/local/usr.sbin.mysqld

Add:

# Site-specific additions and overrides for usr.sbin.mysqld.
# For more details, please see /etc/apparmor.d/local/README.
/run/mysqld/ r,
/run/mysqld/** rwk,

Then:

sudo service apparmor reload 

sources: http://2bits.com/articles/reduce-your-servers-resource-usage-moving-mysql-temporary-directory-ram-disk.html, https://blogs.oracle.com/jsmyth/entry/apparmor_and_mysql

if you dont have apparmor or selinux issues, but still get errorcode 13's:

mysql must be able to access the full path. I.e. all folders must be mysql accessible, not just the one you intend in pointing to.

example, you try using this in your mysql configuration: tmp = /some/folder/on/disk

# will work, as user root:
mkdir -p /some/folder/on/disk
chown -R mysql:mysql /some

# will not work, also as user root:
mkdir -p /some/folder/on/disk
chown -R mysql:mysql /some/folder/on/disk

You should edit your my.cnf

tmpdir = /whatewer/you/want

and after that restart mysql

P.S. Don't forget give write permissions to /whatewer/you/want for mysql user