I have a ~1.5-2mil page views per day site running on 2 servers. One for mysql, other for everything else. Mysql box has a load of 3, frontend is usually 0.0-0.1. Both are dual quad core with 8GB ram running SAS drives in raid5. CPU is idle for majority of the time, iowait is non-existent.
Im running nginx, memcache, and site is built on php. Half the time everything runs perfect, while at other times it lags something severe, when it takes 10-15 seconds for a page to load. Page execution time is always super low, but it seems to hang, waiting for something before it actually loads the page. Whats even more weird is that it only happens to 1 file on the site (but its the one thats most commonly accessed, that actually loads the content on the site). Other pages are super fast at all times, even when it takes 15 seconds to load actual content.
I have nginx_stats plugin installed, and if I monitor it, the lag spikes happen when the write column starts going above 100, and it frequently does... all the way to 500-1000.
It does so at totally random times... not when traffic is heavy... it can do this in the middle of the night, and work perfectly at 5pm when traffic is at its highest.