[Linux-cluster] High availability mail server

[Rick Stevens]

Ray Burkholder wrote:
>> High avail. Mail? That's what MX records are for.  Performance, would
>> be a side effect of multiple MXs.  Having it "clustered" wouldn't make
>> mail deliver any quicker.  Why make something so simple into something
>> complex?
>>
> 
> Mail delivery and MX records are easy.  But once mail is received, you have
> to get it to user's mail boxes, and users have to gain access to the
> repository.  The repository should be 'highly available' in some fashion:
> partitioned storage units, redundant storage, replicated storage, backup
> storage, or whatever.  I believe that is the hard bit:  making the
> repository 'highly available'.
> 
> How do people do it?

It rather depends on which services you're going to offer.  An NFS
volume as a mailstore will work, but doesn't play nicely with multiple
POP3 servers accessing it, and it's slow.  Lots of POP servers rewrite 
the entire mailbox when a user logs in (copy it to a working file, futz
with that, then rewrite the original when they log out).  With IMAP it's
OK but still not ideal.

The primary problem is the lack of file locking.  GFS can address this,
but only if the servers recognize locks on the file system (many POP
servers have to be recompiled to use this, qpopper being one).

If the only locking they do is purely in-kernel and won't deal with a
lock manager or filesystem-level locks, then multiple machines will
collide on the mailstore and there really isn't any way around it.

My recommendation...get and install Cyrus IMAP and POP services, use
GFS and a good lock manager (e.g. DLM).  That should work.

(And yes, I used to manage a system with about 75,000 active users and
over 2M mail accounts--I feel your pain).
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- Rick Stevens, Systems Engineer                      ricks at nerd.com -
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