This post is a result of a lot of frustrated research. I have found several posts about this topic ranging over the last 6 years or so, and no really satisfactory solutions. I hope to collect all the possible ways of synching two Zimbra installations and invite comment from the community. Maybe (hopefully!) I'm missing something.
While there are a number of solutions for High Availablilty / Disaster Recovery (active/passive) which work over a LAN, there are no officially supported solutions which work over a WAN, mainly due to high bandwidth required to sync up terabyte sized installations.
Here are the solutions I've identified, with a brief description, and the inherent problems.
1) Use VMware's HA capability. Apparently you need at a minimum Vmware Essentials Plus Kit. Two physical servers are clustered together within Vmware, and the VMs can live on either (or both) servers.
Problem: not realistic over a WAN, due to high bandwidth required
2) Use Red Hat Cluster Suite. This you have to purchase Redhat and the cluster suite, which is about USD 1500 per server per year. This option is now no longer officially supported since version 8 of Zimbra.
Problem: not realistic over a WAN, due to high bandwidth required; not officially supported.
3) Sync the SAN on a block level between datacentres. I'm guessing you'd need the same SANs at both ends.
Problem: Would need a lot of bandwidth and SAN is expensive; hard to find same SAN at both ends if using commercial datacentres.
4) Use OS's DRBD (Distributed Replicated Block Device) functionality to replicate file systems.
Problem: Again data replication bandwidth is problematic. May be ameliorated by using DRBD proxy. This is extra software at a one time fee of USD 2000. Not officially supported.
5) Use zimbra's replaylog functionality and some community supplied scripts.
Problem: Not officially supported. Some problems reported with missing data; some issues with switching back from secondary to primary afterwards.
6) Use Ubuntu clustering File System. GlusterFS.
Problems: Same as DRBD. Bandwidth. Not officially supported.
7) Give up and put both servers in the same datacentre!
So I'm finding it hard to believe this is the case. Exchange and Notes will do this sort of thing out of the box, and indeed it seems like such a basic function for an enterprise mail system to be lacking.
Why is it not just as simple as doing an rsync over ssh of the /opt/zimbra/store followed by an LDAP sync? And maybe a mysql replication. Surely that's all that's needed for an active / passive setup? Or LDAP sync plus IMAPsync of all mailboxes?
Any help, comments, suggestions and reality checks gratefully received. At the least I hope this serves as a current roundup of the options available for syncing Zimbra. Maybe it should become a wiki page.