Fault tolerant Informix-Online based Unix systems?

We now have a 24 hour, high user system. We are about to upgrade to 7.1. We use use the latest SCO Unix (can't remember the number!). My question is this : We want to try and eliminate downtime. Is there a hardware / software confuguration which would provide this? In other words if a disk drive, ram chip, disk controller, etc. goes down on master system A, another system (system B?) invisibly kicks in, seamlessly to the users, and informs the operator of the hardware error. I'm guessing this would have to at least involve disk mirroring and I'm told it's better to use "hardware mirroring", as opposed to 7.1 mirroring? I'm guessing it could never be perfect. For example if we got a long transaction error on system A then, by virtue of the wonderful mirroring, we would also have it on system B! Any comments, as ever, gratefully received. :)