Managing IMAP

Hardware Note

As with any other service, the hardware demands of Cyrus IMAP can be grouped into demands that are general to any IMAP server and those that are specific to Cyrus IMAP. A good rule is not to just throw hardware, and therefore time and money, at your IMAP server. Find out what kind of appetites your server will have before you feed it.

IMAP servers have an appetite for disk I/O, memory, and network bandwidth, in that order. More specifically, Cyrus IMAP has a ravenous appetite for disk I/O bandwidth. The bigger highway you have between your CPU, memory, and disk, the better. To that end, a good Cyrus IMAP machine is one in which user mailboxes are equally split by load between two or more storage channels. The ideal Cyrus IMAP server has two network interfaces and siphons off all non-IMAP overhead traffic (such as networked tape backups) to one interface, leaving the other free to handle IMAP traffic. It also has more than enough memory to handle peak loads.

Chapter 16, Server Performance Tuning , covers the details of optimizing an IMAP system, but let’s talk briefly about each of these general requirements now. As with any Internet service, memory is of paramount importance. The less paging your system has to do, the better. A good rule of thumb is that your machine should have a bare minimum of 1 MB^[30] of physical memory for each active IMAP process. If you assume one process per user and an overhead of about 20 to 30 housekeeping processes on a black ...