Production servers HD backup
I administer several web hosting (combined with mail relays and other services) production servers under Debian GNU/Linux. I began giving these public services two years ago via three boxes: the first is a gateway which controls traffic via iptables (it's attached to a DSL modem) between a public subnet (the DMZ) and a local network which connects several workstations. In the DMZ subnet I maintain two Pentium-III era boxes, they've grown in services since I set them up. Actually, I think I should buy new ones, but, you know, I want to save money and lenghten its life.
So, they've grown in data hosted, but I've never implemented a resilent backup system. I've set up some rsync tasks sheduled via cron jobs to copy the entire UNIX file system in each of the DMZ boxes, but I'd like to be prepared before an unexpected "real" crash of some HDD, I mean, some problem that renders a disk unusable.
AFAIK, sysadmins sync entire HD backups which are capable of recovering a system via swapping the unusable unit with the backup unit. Maybe the best fashion is to implement a RAID mirroring the unit, I'm I right? So, keeping my systems as they are, I mean, capable of using 4 parallel ATA units, what would you do? Use dump, rsync or some other way to have an operational second unit with an exact copy in a bootable second drive, in order to quickly swap it if the main unit fails?
Comes to my mind to partition a second unit (so making it bootable) and backup daily via rsync only those parts of the unix file system hierarchy which are necessary to boot a system properly. What do you think about this workaround?
I'll appreciate your comments, cause I don't really know what is the best way to achieve this... Thank you!