RAID easy disk replacement for datacenter employees

[Jon]

I'm about to send an OpenBSD server to a datacenter for a client and we 
need RAID in case a hard disk fails. I need answers from people who have 
real world hands-on experience and can tell me what to use so that, if a 
drive fails all that's needed is a datacenter employee to walk over, 
pull a 3.5" out of a 5.25" enclosure behind the door of the 4U case and 
insert a new drive while leaving the system up and running.

If the drive fails during or between a system restart, which RAID 
controllers that work with OpenBSD will simply use the working drive and 
continue booting/running as normal? Is there hardware that will handle 
writing the data to a replacement drive automatically or can be 
interfaced with to do so with a shell? What RAID hardware can give 
userland programs the status of the drives?