Backing-up backups ?

I've got Zextras Backup running on a Zibmra8 server.

It's set up to run a mixed schedule of Full & Smart backups daily @ 4am.

They're stored to /opt/zimbra/backups/zextras/... on the Zimbra server.

I now want to backup a fully-recoverable instance of Zimbra to another box, @ network storage.

What's the best method/procedure?

Do I just rsync the Z8 server's /opt/zimbra/backups/zextras folder, or all of /opt/zimbra?

In either case, do I need shutdown the Zimbra server to get the mysql data (via a cold backup?)

THanks