Number of used connections soar high every 30 minutes

[Julien Walther]

Hi, all,

maybe the problem has nothing to do with mySql at all, nonetheless I will try to seek some advice here.

We are operating a fairly large online-community, running on php/mysql/apache. We have three webservers, one db master and two db slaves.

Recently we are trying to figure out one of the weirdest behaviours ever noticed:
Every 30 minutes, to the full and half hour (absolute accurate world time), the simultaneously used connections of our DB master go up from an avg. of usually 12 to 800 or more, make the whole website almost unusable for a few seconds. Then it goes back to normal load.

We have stopped ALL crons for testing; the problem still remains. There is no script running at this interval.

There must be any internal or external cause for this problem. But even a high-performance (malicious) apache bench or the like will not cause such a traffic.

Does anyone have the slightest hint on what this might be?

Thanks a lot in advance.

Julien,
Hamburg/Germany