For most data an rsync like solution would work just fine. The problem with that is with files that are constantly being written to and changed like database files. Partial data writes during an rsync can corrupt the mirrors. Of course this is true of any thing but rsync would be slower having to periodically scan the drive for any files that have changed and may not sync files that currently open, locked and being written to. Using a lower level of sync could allow the data to be changed simultaneously on many systems without delay.
I have follow this thread for a while and I think we are missing some obvious solutions here. Either that or I have just over looked them.
You need to break the hardware system from the data in your mirroring.
The data could be written to an external raid array with multiple interfaces. The raid array handles the information redundancy requirements and can be accessed by at least 2 machines.
The machines run the exact same software which could be rsync'd to keep the same or could share system files from the raid array. One machine monitors the other machine using a heartbeat monitor as suggested by Maurice. (The second machine assumes the first machine's IP.)
If one machine fails the other takes over and still has access to the same files. If a drive fails, your raid array doesn't
go down.
For full redundancy you would want a raid array with redundant power and controllers.
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