Server Migration -- ZCS to ZCS with no/minimal downtime
Want to run something by those who are seasoned Zimbra administrators...
I'm currently in the process of planning a migration from our current ZCS NE 6.0.x deployment (CentOS 5.6 x86_64) to a new server hosted in a new location of ours. One of the important factors for this project is to minimize downtime as much as possible, since our single-server deployment currently runs email + calendaring for the entire company (~1000 accounts). Additionally, we're totaling about 3.5TB of content (no quotas).
Because we have 3.5TB of email (etc), shutting down Zimbra and performing a massive rsync/scp to the new system isn't possible as we'd have too much downtime. There's a 100Mbps link between the two sites; would just take too long for our comfort. So, I've been investigating alternative migration practices.
One of the ideas I came up with was to install Zimbra on our new server and join it to our current system as a replica LDAP host and a new mailstore. Then, at my leisure, I could use 'zmmailboxmove' on each account over the course of a week or so, to move each account over to the new system. Our current server would of course have to be reconfigured to be a Web as well as IMAP/SMTP proxy, but from my experiments with Zimbra, this was a rather trivial process and really was only a matter of stopping/starting Zimbra along with a couple commands.
Once all mailboxes have been migrated to the new server, I would stop the old one that now has no remaining users, and promote the new system to become the LDAP master. I'd also adjust DNS at this time such that the new server's IP is officially our mail system and no longer a hidden Zimbra server node behind the old server's proxy services.
What do you think of this migration procedure? Bad idea and you foresee many difficulties on the road ahead? Or have folks used this method time and time again with success?
Or... do you have any other methods of migrating 3.5TB ZCS instances with the least downtime possible? Perhaps doing a backup/restore onto the new server and during our scheduled maintenance window, replaying redo logs?
Any tips and war stories would be most helpful.