I do log rotation once per day at night. The cron job is as follows:

1. cat /etc/cron.d/mongodb
   05 0 * * * root [ -f /var/run/mongodb.pid ] && /bin/kill -USR1 `cat /var/run/mongodb.pid`

Approximately once per week the mongod process hangs after the log rotation:

• It renames log files,
• the new mongodb.log file is empty,
• it's impossible to connect to the server (it prints the header with version however)
• the only way to stop it is to send KILL