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Well, if it gives you any confidence we have done almost exactly
what you are thinking about doing and it works great. You should
take what Josh said to heart though because these are problems you
will have to deal with. During the process of building ours the by far
the hardest part was the shared storage part of it. You will need some
pretty beefy hardware to handle the load. Choose your filesystems
wisely. I can tell you right now that ext3 melts down when directories
start getting large as they sometimes do with the Maildir format.
Steve
Josh Tolley wrote:
ed wrote:
Hi, Im very new to clustering. I have perhaps a very simple question:
I have to provide a email service, but with low spec computers which
we have in abndance. The service is Qmail, which we currently have
running on three boxes, each with different IP address and each have a
seperate MX record.
Now, I assume that the email service running behind a LVS will use a
single MX IP address x.x.x.x.
Right... your director (the load balancer) will probably just have one
IP address available to the external world.
However, I have not got a clue how to go about this. Should I aim to
have two single computers linked via a serial interface (for heart
beat), or should I do something more adventurous such as link them via a
network interface.
Depends on what your goal is -- see below.
We have 4,000 domains to look after, currently we just do forwarding,
which is not at all intensive. Once the spam and virus scanning kicks in
its a huge latency on the mail, scanning takes about 1.3 seconds per
mail, with the current load of four domains (which catch all). I hate to
think what this will be like with the other domains also.
My main objective with this is to allow myself to add computers on the
fly to the LVS pool so that I do not run the risk of bringing down the
network of email boxes when something needs an upgrade.
So it looks like your goal here is to provide failover and to split up
processing load between computers. Sounds reasonable. The biggest
problem you're probably going to have is shared storage. If you have,
for instance, users retrieving mail via POP3 and sending via SMTP, a
user connecting via POP3 to one real server will have to be able to
get mail that was delivered by another real server. So you probably
need somewhere to keep the mailboxes that all real servers can get to.
If you use qmail's Maildir format, which works over nfs, you can set
up an nfs server, but 1) it *might* have a heavy load, and 2) it's a
single point of failure. You'll also probably want something to
replicate settings between your real servers (qmail's control files,
spam and AV configuration files, mailbox config info, etc.) That will
probably take a whole bunch more work than just getting the LVS part
working.
Josh Tolley
Raintree Systems, Inc.
http://www.raintreeinc.com
760 509 9000
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