I don’t think thats going to work. To do a full proper sync of the data, you need to be able to have the mail server in a stopped state to copy all the data.
If you were do to that and then bring up the slave server, you’d run the risk of some corruption and certain emails / calendars / contacts potentially not being there or being corrupt. You might also run the risk of have X amount of email clients start to re-download the data again.
You’re best bet to do this would be to have some spam filter sitting in front, and cache incoming emails until the main / slave servers are back online. You might even be able to get a caching server that can deliver emails to both server simultaneously.
External email → Firewall → 3rd party spam filter → Delivers to Server 1 and Server 2 at the same time.
You’re only downfall would be on the primary mail server you’d need to setup a filter that sends everyones emails to their sent folders on the slave server so a copy exists. Might not be ideal.
The other option would to copy your full data to the slave server
nightly stop the primary server
run your rsync to the slave server
bring up your primary server.
It should sync fairly fast in reality. Your data stores and mine are about the same time. For my to sync my first full copy over 10GBe, took approx 5 hours using this command:
rsync -aHx --numeric-ids --progress --delete -e 'ssh -T -c aes128-gcm@openssh.com -o Compression=no -x ’ /var/opt/axigen/ root@10.10.10.230:/var/opt/axigen
Subsequent emails during the day differ in length depending on which DB files in the axigen folder get touched. At the end of the night without stopping the mail service, it takes between 1-3 hours.
I’m contemplating doing just what you do, but currently all my servers are setup as VM’s on the High Availability storage and servers, so if any of the main host servers fail, the mail server will fail over to another within a few minutes and continue to run.
The biggest issue I have would be a complete back end failure on a piece of hardware, which is where this exact process would excel. The issue is that the axigen service needs to be completely dead before you can do a final successful sync.
For my existing backups, I used the axigen backup script that I have mounted on my server from a different storage array thats setup in LVM and then I have borg that runs backups to another storage away that I maintain backups for basically forever.