1. Moodle Scalability What is Scalability?
performance, code maintainability, fault tolerance, and the availability of programming staff.
you add users, you add more servers

2. a Dell dual 2.8Ghz
3GB of RAM
100 courses with ~1800 active students, average students/course though one has1047.

3. http://campus.openpolytechnic.ac.nz/moodle/ server cluster
6000 courses
27,000 students
Source:

4. 1 Dual Opteron + 4GB RAM +
Plenty of SCSI disk space running Postgres, LDAP and NFS (for moodle's data directory)
System logs, DB logs and DB storage are segregated to separate drives each for maximum concurrency, all the shared memory
a lot of monitoring of performance wrt DB queries, io, swapping, etc.

5. 2 webservers running apache+php+cronjobs -- cheap Dell 1U rackmountables
1 load-balancer that runs at the tcp layer

6. Moodle Benchmarcking framework
Given the needs (number of courses and users), to be able to compare different servers and configurations.
good logging of resource to identify which pages are performance hogs. This helps on knowing where we want to focus our optimization efforts for maximum effect.

7. Moodle Benchmarcking framework
Web Application Stress Tool
The Microsoft WAS web stress tool is designed to realistically simulate multiple browsers requesting pages from a web site. You can use this tool to gather performance and stability information about your web application. This tool simulates a large number of requests with a relatively small number of client machines. The goal is to create an environment that is as close to production as possible so that you can find and eliminate problems in the web application prior to deployment.

8. Moodle Benchmarcking framework
Apache JMeter
Apache JMeter may be used to test performance both on static and dynamic resources (files, Servlets, Perl scripts, Java Objects, Data Bases and Queries, FTP Servers and more). It can be used to simulate a heavy load on a server, network or object to test its strength or to analyze overall performance under different load types. You can use it to make a graphical analysis of performance or to test your server/script/object behavior under heavy concurrent load.

9. Moodle Benchmarcking framework
JMeter works by taking an initial capture. You set it to act as an HTTP proxy, you just run traffic through it, and it captures all the requests (POST/GET, with data, cookies, everything). So you set it up and get a group of real users to use Moodle as they would normally. Create new courses, edit content, read forums, post to forums, etc. Once you have this "traffic profile capture", you tell JMeter to replay it against the server. It will do it as fast as possible, and with high parallelism.

10. Wikipedia Servers