[Dovecot] Load Balancing and HA

Romer Ventura rventura at h-st.com
Wed Jun 5 18:23:55 EEST 2013

> On 31-05-2013 12:57, Patrick Westenberg wrote:
> > Romer Ventura schrieb:
> >
> >> Scenario1: This should allow any to lose any of the servers and
> >> clients still have access to their emails (although I am not sure how
> >> the indexes would react to this and sudden disconnection)
> >>
> >> -          2 Dovecot Proxy servers, using a virtual IP to where the
> >> clients
> >> will connect to from the WAN and LAN
> >>
> >> -          2 Dovecot+Postfix servers with local cache
> >
> > Your proxy won't reconnect a user to backend B if the backend A fails.
> But doesn't that depend on how the vitual IP is managed ie what kind of
> system is behind it?
> For example a simple heartbeat setup would correct this at the cost of one
> machine sitting idle.
> Other setups using load balancers can correct this.
> There is a better solution using the Director service of Dovecot where users
> are assigned to one of several bacend machines and disconnected when idle.
> Even, there is a script that monitors the health of the Director backends and
> adjusts accordingly (which I haven't personally tried yet). See
> http://wiki2.dovecot.org/Director
> http://www.dovecot.org/list/dovecot/2010-August/051946.html

Well, I am successfully using Ucarp with apache and Mysql to handle the IP handover if a host is down. I also seem to remember reading that using dovecot proxy and deliver, it can send the user to a different backend if any of them are unreachable. If this is not possible, using keepalive/ucarp (since they are simpler than heartbeat) would solve this.

I try to stay away from clustering specially since XenServer cant do direct LUN to VM like VMWare can and to minimize the painful split-brain issues we would have to add a 3rd server for quorum... 

We don’t have a large user base, but being able to shutdown do maintenance on a server during business hours is a plus. We do have a lot of traffic for oure user base, we see around 200K emails per week.

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