Rudi Ahlers wrote:
Sorry guys, I want to stick with a SMTP / IMAP / POP3 server, not webmail. I'll be using Horde for webmail as well though
there are two general classes of clustered systems, high availability, and high performance.
HA clusters are usually active/standby, and might use stuff like heartbeat, drbd, etc.
HP clusters are either load balanced, or active/active... Things that demand ACID (Atomicity, Consistency, Isolation, Durability) like databases, mail servers are very complex to cluster this way on a active/active (aka multimaster) environment while maintaining the integrity and a reasonable performance. just implementing load balancing does not by itself provide any redundancy in case of component failure. A simple load balancing scenario for a mail server might be having one server to handle all internet mail incoming and outgoing, while another server handles local users reading their mail (eg, pop or imap)
its best to define your requirements and expectations before diving into these waters, as clusters can be far more complex and intricate to configure and administrate than discrete systems.
re: mail servers specifically, there are two seperate classes of storage that would need replication... One is the mail spools and queues as used by the MTA (postfix, sendmail, etc), and the other are the user mail folder(s) as used by the local delivery agent (procmail or whattever), and read by the mail client (pop, imap).
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