"Outside The Box" OSS Backup Solution
Although we just upgraded to the Network Edition, we backed up our previous OSS installation using a completely different method.
First, we installed Zimbra in a VMware Server virtual machine. (I expect you now see where I am going with this...)
Next, from the VMware host, we would just suspend the Zimbra virtual machine, copy the Zimbra virtual machine files to our desired backup location, and then resume the suspended Zimbra virtual machine.
To restore the Zimbra virtual machine to a different host, you'll need a copy of VMware Workstation, because VMware Server won't create the new keys required when you move an existing VMware virtual machine and want to fire it up on a different VMware host.
Our Zimbra installation was about 5GB, which didn't take too long to copy. And yes, although the Zimbra server was effectively "down" during this backup procedure, no shutdown/restart was required.
This procedure gave us full, in-sync backups which we would then move off site.
For on-site backups, we used VMware Server's "Snapshot" feature while the Zimbra server was running. We tested restoring twice and got no errors.
One of the Zimbra employees pointed out that having a restorable backup means keeping ldap, mysql, cyrus, and postfix in sync. Having built a number of mail servers using these fine software packages, I can personally attest to the fact that doing a restore when these packages' stores are out of sync is a long, long process that is not assured to recover 100% of the data backed up.
So, that's one of the key reasons why (after testing the OSS version to our satisfaction), we opted to buy the Network Edition.