tmpdir permissions problem?

[Stephen Worthington]

I am running mysql 5.5.49-0ubuntu0.14.04.1 on a Mythbuntu 14.04 system. Yesterday, I ran into a problem where my system partition was too small for the temporary files created when the MythTV optimise_db script runs, which does REPAIR TABLE and OPTIMIZE TABLE commands on all tables in the MythTV database, including the huge recordedseek table. So I thought it should be simple to change where mysql puts its temporary files, by altering the tmpdir setting. The current tmpdir setting (in /etc/my.conf) is:

tmpdir = /tmp

and I overrode that by adding a new tmpdir setting in /etc/mysql/conf.d/mythtv-tweaks-jsw.conf file:

tmpdir = /mnt/rec3/tmp

When I restarted mysql, it failed to start and this was in /var/log/mysql/error.log:

/usr/sbin/mysqld: Can't create/write to file '/mnt/rec3/tmp/ibayJP7i' (Errcode: 13)

The /mnt/rec3/tmp directory exists and has the same permissions and ownership as the /tmp directory:

# ll -d /tmp
drwxrwxrwt 7 root root 12288 May 14 22:06 /tmp/

# ll -d /mnt/rec3/tmp
drwxrwxrwt 3 root root 8 May 14 15:50 /mnt/rec3/tmp/

and the /mnt and /mnt/rec3 directories also have full permissions:

# ll -d /mnt
drwxrwxrwx 31 root root 4096 Apr 23 00:10 /mnt/

# ll -d /mnt/rec3
drwxrwxrwx 11 root root 4096 May 14 14:25 /mnt/rec3/

So does anyone have any idea why this does not work? And how I can get mysql to use a different tmp directory?