John R Pierce wrote:
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).
_______________________________________________
Ok, I see where you're going, and a bit of clarification is needed :)
I need a simple failover type cluster, where any 1 of the 2 machines
currently in the "cluster" can handle anything. The client (a small
rural clinic) is 700KM's away, and they do have frequent power failures,
so bad that even the UPS' lifespan has shorten. This is a donated
project, so funds (and hence equipment / reliable equipment) is limited.
We currently have 2servers with Dual Core PIV + 2GB RAM + 2x 160GB HDD's
each. The HDD's is setup on RAID1, and the two servers replicate MySQL
on an active/active (Master - Master) replication. The intranet site &
file server data is being replicated via rsync. This all works well, but
I need a mail server.
The main server will store all the emails (like an exchange server
does), the users will use IMAP (so, a) it's backed up & b) they can
always access their email from any workstation) & SMTP, with the ability
to use POP3 from home / on the road. It would be nice to have a shared
calendar & address book as well, so something like phpgroupware / open
exchange / etc would be nice.
But, I don't need load balancing, just high availability
--
Kind Regards
Rudi Ahlers
CEO, SoftDux
Web: http://www.SoftDux.com
Check out my technical blog, http://blog.softdux.com for Linux or other technical stuff, or visit http://www.WebHostingTalk.co.za for Web Hosting stuff
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