When is it necessary to restart nginx and reload will not suffice?
Does it make a difference if an extension like passenger is used?
Should the service be restarted if it consumes too much memory. Any other reasons for restarting Nginx, particularly after a configuration change either in an extension or a Nginx core config?
After making a configuration change, one can either restart or reload nginx, via the binary itself or the init.d script "/etc/init.d/nginx -h" on Ubuntu. Which method should be preferred?
Reloading nginx is safer than restarting because before old process will be terminated, new configuration file is parsed and whole process is aborted if there are any problems with it.
On the other hand when you restart nginx you might encounter situation in which nginx will stop, and won't start back again, because of syntax error.
Reloading terminates the old process, so any memory leaks should be cleared anyway.