We are trying to set up a pair of web servers with identical hardware and configuration. We want to keep the servers synchronised where possible by swapping disks between each machine's RAID1 array of SATA drives (the four drives are identical).
That's a lot like our setup here. Our primary server has a RAID 1 array. Our backup is a stripped down version. (only one hdd, less ram, and a slower processor).
I have an rsync script sync the websites 4 times a day and the MySQL databases are synced every hour. (MySQL information changes a lot more then actual web site content)
The backup server has a tape backup drive that does a full system backup each night. Our tapes are on a 7 day rotation.
If a hard drive dies we're fine,
If the primary server dies we're fine,
If the server gets hacked, and both servers are compromised
(problem could autmotically replice to mirror)
Then we can restore from tape.
Worst case scenario is we lose 1 day's worth of data. For some companies that's unacceptable, but our web server doesn't hold a lot of highly critical information.