[CentOS] clustered mail server?

[John R Pierce]

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).