How Do You Backup Hosting Server

we going to running a hosting company

but not sure how to manage the backup server

Server A = 10 host accounts
Server B = 15 host accounts
Server Backup use rsync to get 25 account

will it be work?

and each domain the name server

ns1.host.com < - Server A
ns2.host.com < - Server A
ns3.host.com < - Server B
ns4.host.com < - Server B
ns5.host.com < - Server Backup
ns6.host.com < - Server Backup

www.abc.com

ns1.host.com < - Server A
ns2.host.com < - Server A
ns5.host.com < - Server Backup
ns6.host.com < - Server Backup

if server A down, so auto point to backup server

so is it work?