[Mailman-Users] Load-balancing mailman between two servers

[Brad Knowles]

At 3:03 PM +1100 11/27/06, Guy Waugh quoted Kim Hawtin:

>>  How/where do you share the incoming mail list aliases that sendmail checks?
>>
>>  Also when you create a new list, how to you update the other hosts aliases?
>>
>  On each server, in the sendmail aliases file. So, when adding or
>  removing a list, I have to do the alias changes on each of the two servers.

Hmm.  With postfix, you can specify multiple alias files, some of 
which will get auto-rebuilt as necessary by postfix, others which can 
get manually rebuilt by other processes (like by Mailman, with the 
standard tools it provides).

It's been a while since I mucked around with sendmail, but I have to 
believe that the same is possible there.  Indeed, I believe that the 
technique that is currently used to completely automate this process 
with postfix was adapted from the technique that previously worked 
only with sendmail.

You could put that second set of aliases in the NFS shared space, so 
that you shouldn't have to rebuild that separately on the two boxes.

>  One thing I am a bit concerned about is contention on the shared files
>  between the two servers. If, for example, both servers wanted to update
>  the same .pck file at the same time, I'm not sure what would happen...
>  would one server lock the file and the other server wait for the lock to
>  be released, does anyone know? Or would chaos ensue?

The developers of Mailman have gone to great lengths to make the 
system as NFS-proof as possible.  That is to say, it should be 
possible to share the entire /usr/local/mailman structure via NFS (or 
wherever you put all the Mailman files, including archives, queue 
directories, etc...).  It should "just work".

That said, file locking on NFS is problematic during the very best of 
times, and I believe that the Mailman developers have gone to great 
lengths to try to work around that.  But, by putting that stuff on 
NFS, you are increasing the chances that you'll run into a situation 
where you get lock contention, or maybe stale locks that will need to 
be cleared out.  You will increase the amount of system maintenance 
that you have to do -- that's simply unavoidable.

And a very great deal depends on your NFS server.  Putting all this 
stuff on NFS can cut your throughput that you can handle by a great 
deal -- orders of magnitude or more, and that's if you've got a 
high-end mega-expensive dedicated NFS fileserver from the likes of 
EMC, Auspex, or Network Appliance.  Just keep this in mind as you 
program for redundancy.


I'm not saying that you can't put things on NFS, or that it's even 
"unwise".  I am saying that everything is a set of trade-offs, and 
you've got to understand what it is that you're trading for what.

-- 
Brad Knowles, <brad at shub-internet.org>

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