So, both MongoDB and Redis are DB's optimized for keeping stuff heavily in memory. I wouldn't use either for heavy logging.

RRDTool is nice, but only for data where you don't mind losing records.

For straight logging, SQL is actually a better solution than a lot of those options. If you need weird queries, something like Elastic Search or Solr on top of your SQL DB can buy you quite a bit of freedom. As can denormalizing your SQL.

One other thing to consider of course, is something like syslog.