As a "3rd-pass" at helping:
Mike said:
I'd recommend tools.percona.com to get you started too. I start with their configs all the time:
You're absolutely wrong. You can't start with a boilerplate my.cnf and hard values for RAM and caching because you have to consider your total RAM first, or your performance will go down because MySQL will run out of memory. Use https://github.com/major/MySQLTuner-perl to determine what you need, walk the values up and down and test.
Disk I/O tends to be the first thing that limits WordPress site speed.
Absolutely wrong again. Look at the disk swap. It's zero right now. What limits WP long before disk I/O is plain old RAM and CPU and database speed.
What is Kate asking about? Not about memcaching or APC, but MySQL maxing out connections. That's MySQL configs.
Yes, APC, etc will help, but if it's a box that's already handling 100 sites, the admin probably already has that configured. And configuring such additions to php is best done on the box and not by using a WP plugin.
Kate: use phpinfo to see what you're got in terms of php configs (APC, etc) right now:
short link to KB article at Media Temple: http://bit.ly/143q7s1
And cache plugins help, of course, but they can't make up for server inefficiencies to begin with. WP Super Cache is far more reliable and easier to configure that W3TotalCache. Search these forums for feedback on both. Super Cache has 4.6+ million downloads; it's the best out there.