1500 concurrent users is not a joke! This is an (incomplete) list of things I would look at:
- To serve each user at say 500 kbit/s (a common bandwidth people are used to in Europe) the total will come upto 750 Mbit/s! OK, nobody uses 500 kbit/s continuously during interactive work, still this rather theoritical calculation gives an idea of the size of the work.
Have a carefull look at your network topology. Can it distribute the required bandwidth?
- Consider that a "good" network card is designed for 1000 Mbit/s. That is also "theoritical", I don't know of a single server in the PC range which could saturate a GBit network card. What about the harddisk subsystem, even if the webserver can process the PHP scripts at that rate?
I would also consider multiple servers as Pieterjan suggested.
- Definitely a separate database server, even a cluster.
- May be multiple webservers have to share the work, say in round-robin fashion.
The starting point is this http://docs.moodle.org/en/Performance
You need much more. There were plenty of discussions and practical tips in this forum. Try the search facility.
Hardware and performance
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