We have successfully been running Moodle 2.3 on a medium large site that we upgraded from Moodle 2.2 on Aug 8, 2012.
The Database size is ~17GB. Google analytics typically reports 200-300 active vistors. Looking in Moodle logs broken into 1 minute periods we hit daily peaks of 150-200 users and 500-1000 logged events. Here are our numbers from our Moodle registration page:
- Number of courses (14817)
- Number of users (68246)
- Number of role assignments (153713)
- Number of posts (130792)
- Number of questions (414304)
- Number of resources (44427)
Besides our current production Moodle 2.3, we also have a Moodle 1.9 site that has courses we offered before moving to Moodle 2 that has DB ~32GB.
We do update Moodle regularly to get improvements, and are currently running 2.3.2+ (Build: 20120927). We have one web server and one DB server. The servers are identical so that in a short time we could fail over from one to the other. The servers are Dell R710's and have 80GB RAM, 12 hyperthreaded cores, 15k rpm local disks, 2x 1G internet connections, 2x 1G connections to good quality SAN for moodle data, RHEL 6.3 with remi and EPEL repositories to meet PHP and MySQL requirements for Moodle 2.3. We are not running virtual machines. I wouldn't run Moodle on virtual machines for a large site unless I was enough of an expert on VM's to not need to ask questions on these forums. ;)
We have spent a lot of time ensuring that our hardware and software doesn't have avoidable bottlenecks. We have followed advice and done research based on the Moodle performance documentation. We have upgraded hardware twice over six years, from Sun V440's to Sun X4600 to are current servers. We have optimized both Apache/PHP and MySQL for our workload. We use the latest version of APC cache. I can't see running a Moodle site of any significant size without a PHP cache. MySQL 5.5 with InnoDB on RHEL 6 is a big improvemnt over MySQL 5.1 with MyISAM on our previous Sun configuration. For anyone using Mysql, I would recommend High Performance MySQL: Optimization, Backups, and Replication, 3rd edition by Schwartz, Baron, Zaitsev, Peter, Tkachenko, Vadim. We have performance monitoring. We do performance testing using JMeter on test servers with similar set-up to production.
We haven't seen the difference Paul Nijbakker saw with "Open in new window turned on in Site Administration > Appearance > Moodle docs", but some performance issues are very site configuration dependant.