Hi, I've got a question about memory usage. I'm not entirely sure what to make of this munin graph:

It's for our mailserver (running postfix, courier-imap, mysql, and apache2 with mod_ssl enabled), and I just don't know what "cache" means in this case. I've noticed that the jumps in memory usage come in regular intervals, usually right around midnight. Could this affect the server's performance in any way?

Thanks for any help offered!