fault tolerance/redundancy/load balancing

[Leen Besselink]

Maybe I can add what I do/think about redundancy/load balancing

[..]

> >system, and would like to redo everything with fault
> >tolerance/redundancy/load balancing in mind between the two servers.  My

[..]

> >
> >
> Mack:
>

[..]

> They're configured to do LDAP routing, each querying it's own directory
> server. The 3 replicated directory servers tell the mail switches what
> address to redirect each piece of mail to. This allows us to hide
> multiple mail servers behind the switches. The various "mailbox" servers
> can be (and are) running sendmail, MS-Exchange, Groupwise, etc. Each
> back-end mailbox server has it's own set of administrators, familiar w/
> the users of that system.
>
> Aside from allowing us to have multiple mailbox servers, in multiple
> locations, the switches (due to the LDAP queries) will refuse to accept
> mail for bogus usernames, so that they aren't saddled w/ the task of
> sending out undeliverable messages to the forged sender addresses of
> every piece of SPAM that comes along.
>
> Each mailbox server (transparently to the end users) is locally
> delivering mail to a subdomain of the "real" domain served by the
> switches. One nice feature here is that the /etc/mail/mailertable file
> can be used to "hide" these subdomains from the rest of the world.
>

If you want POP/IMAP also to be transpart to the user I recommend:
perdition a pop3/imap proxy with LDAP-support (I'll start using it in
production in a few weeks).

> For example, mail for the domain called subdomain1.mydomain.com might be
> delivered locally by the server mail1.mydomain.com. The data in the LDAP
> directory would cause the switches to re-address mail for
> user1 at mydomain.com to user1 at subdomain1.mydomain.com.
>
> Someone recently asked a question about sharing a single quarantine
> directory between a pair of machines running mailscanner. My only
> suggestion was to schedule a file-transfer to a third machine, so that
> no failure of  single machine would deprive either mailscanner of a
> quarantine filesystem.
>
> I have yet to see a really good "CPU/cabinet level" solution for
> redundancy of a mailbox server's  message store. For a while I was
> playing w/ DRBD, but found it to be unstable on dual CPU machines. For
> drive redundancy you've already got RAID-5.
>
> Did I miss anything? I can't think of any other redundancy issues.

What I'd love to see is some kind of distributed filesystem, where all the
mailboxes are stored, so you'd get something like this:

- 2 fileservers (with a replicated distributed filesystem)
- 2 mailservers (with IMAP/POP3 + SMTP + Maildir + MS + SA)
- all configuration stored in LDAP
- replicated POP-before-SMTP

This can be expanded where you'd have 2 mailswitches, which do the SMTP +
MS + SA (and expanded on that by just using a larger number).

There are still 2 parts of this setup that I'm missing:

- replicated POP-before-SMTP - I've not found a simple/easy solution for
this

- 2 fileserver with distributed filesystem - I hear some people use
Network Appliance, but personally I'm still looking at lustre.org (they
should be ready in 6 months or so) or similair (there are a lot of
similair products out there) - in the mean time I'm gonna use perdition
and store the Maildirs on the POP/IMAP-servers.

This is how far I've got it till now.

___________________________________________________________________________________
Things should be made as simple as possible, but not any simpler. - Albert Einstein

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