Managing IMAP

Kernel and Network Driver Tuning

System tuning or, more specifically, kernel, driver, and filesystem tuning are “lady or tiger” propositions. The best advice is to embrace conservatism. Before you dive into any wholesale system tuning, take some snapshots of how your system is performing over the course of a week or so, make some modest changes, then watch it for another week or so. All too often, system administrators change 5 or 10 independent variables at a time, then fall prey to the fallacy of false cause, assuming that it was one particular that caused that huge increase in system performance. Of course, it might just be that it’s December 20th and most of your user base just left on Christmas vacation.

Diagnose the Problem

You’ve got a comfortably low system load. You’re barely paging at all. You’ve lots of disk space, and your disk channels are not I/O bound. Your IMAP server performs wonderfully when you crank up PINE on the server itself and point it to localhost as the default IMAP server. Everything ought to be great, but it isn’t. You’ve got a steadily growing queue of users all complaining that IMAP performance has dropped to its knees. Sounds like you have a networking problem on your hands.

This sounds like a job for netstat. netstat is one of those gifts that come with all Unix systems. Actually, most systems with good TCP/IP support come with netstat. Even MS-Windows has it.

Using netstat

Provided you’ve got connectivity between your clients ...