=== Try SET GLOBAL plugin_dir ===
Error: Variable 'plugin_dir' is a read only variable

=== MySQL data dir ===
/www/server/data/

=== Can we write to data dir via general_log? ===
Set: datadirtest_write.txt
Match: YES
Write: FAIL (maybe open_basedir)

=== Check if MySQL can load plugins from /tmp ===
datadir: datadir
plugin_dir: /www/server/mysql/lib/plugin/
Same filesystem? UNKNOWN