[Dovecot] remote hot site, IMAP replication or cluster over WAN

Johan Hendriks joh.hendriks at gmail.com
Wed Nov 3 10:32:59 EET 2010

Op 3-11-2010 7:17, Stan Hoeppner schreef:
> Stefan G. Weichinger put forth on 11/2/2010 1:15 PM:
>> A bit off-topic, sorry ... I want to set up a hot backup dovecot in a
>> VM, aside the physical server, so I am very interested in the "best
>> practise" to do so ...
> There isn't one.  If there was Timo would have pointed you to the wiki.
> Doing server fail over is inherently problematic for a large number of
> reasons.  The easiest way to implement it is to literally turn on the
> backup server (power on) when the primary fails.  The backup comes up
> with the same hostname and IP address as the primary and mounts the same
> physical storage.
> The storage must be either a SAN LUN, NFS directories, or a local disk
> that has been mirrored over the network during normal operations.  But,
> you can't have the same hostname and IP if the machine is running
> allowing the mirroring to take place.
> Thus, for a "standby" server, it must be powered off and take ownership
> of the storage when powered on.  You _can_ do realtime mirroring to the
> standby while it's running, but then you have some really complex issues
> to deal with as far as hostname and IP assignments when the primary host
> dies and you have to take over the name and IP on the spare server.
> This can be done with a reboot and using alternate config files, and
> might actually work better in a virtual environment than with a physical
> machine as VM guests tend to boot faster than physical hosts due to
> things like long pauses caused by hardware BIOS routines.
> The key to all of the above is proper identification of primary host
> failure.  The biggest problem with this setup is the "two brains" issue.
>   There are a number of network scenarios that can cause your backup
> server or monitoring software to think the primary host is offline when
> it's really not.  The secondary thus comes up, and now you have two
> hosts of the same name and IP address on the network.  This situation
> can cause a number of serious problems
> IMO, the best way to do high availability is to use an active/active
> cluster of any number of nodes you see fit to meet your performance and
> reliability needs.  All hosts are live all the time and share he load.
> When one goes down client performance may simply drops a bit, but that's
> about the extent of the downside.
> It's inherently more straight forward to setup than the previous
> scenario, especially if you're using NFS storage.  In this case, you'd
> build two identical Dovecot servers and have each mount the same NFS
> mail directory.  Read the list archives for ways to mitigate the index
> file issue.  Timo wrote a new director specifically to meet this need.
> Two other options for the shared storage are a fiber channel or iSCSI
> SAN, or using DRBD to mirror disks (or logical devices--RAID) over the
> network.  Both of these solutions require using a cluster filesystem
> such as GFS2.  These can be quite a bit more difficult to setup and get
> working properly than the NFS method, especially for less experienced
> sysadmins.  They can also be more difficult to troubleshoot, especially
> for sysadmins lacking sufficient knowledge or aptitude with regard to
> storage hardware and low level Linux device drivers.
> Hope this helps you a bit.  You probably won't find a "how to" document
> that spoon feeds you the steps for an exact build/setup of this.  If you
> choose the DRBD route you might be able to get Eric to write you up a
> step-by-step of how he did his two node DRBD Dovecot cluster.  Maybe
> he's already written one. :)
Hello, i am working primarly with FreeBSD, and the latest release has a 
service called HAST.
See it as a mirrored disk over the network.
You can install both machines with dovecot, and use the hast disk as the 
data storage.
With CARP in the mix, when the master machine fails, it starts dovecot 
on the slave.
This way you have a failover without user interference.

I did not test it, but i hope when time permits, i can try to start 
testing this setup.

Johan Hendriks

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