Managing IMAP

Memory Tuning

How Much RAM Is Enough?

Figuring real memory requirements is a complex task and is somewhat platform dependent. On IMAP systems, the physical memory requirements are directly related to the number of users actively reading their mail at one time. IMAP processes are long-lived and typically inactive for long periods. For example, a user may spend 10 minutes reading and replying to a message during one connection. That being the case, there’s no reason why an IMAP process should be held in physical memory for the duration of the connection—it’s perfectly acceptable for the process to be swapped out to the swap space.

A survey of large sites that run UW or IMAP servers shows that an IMAP system optimally requires 1 to 2 MB of physical memory per active IMAP session. For UW servers, the 1 to 2 MB rule also applies, but it’s advisable to steer towards 2 MB. A UW IMAP process will grow to accommodate the largest message in the mailbox, so a 40 MB message will result in a 40 MB or larger IMAP process. For Cyrus servers, 1 MB per active server is sufficient, provided you optimize your swap space.

Optimizing Swap Performance

Your swap configuration is every bit as important as the amount of physical memory on your system. Swapping occurs when the operating system moves an inactive process from RAM to disk, freeing up RAM for active processes. Here are some guidelines for configuring your system’s swap space for ultimate performance:

• Put swap space on a separate disk partition ...