RAID decides two things about your server before it ever boots: how much of the storage you're paying for is usable, and what happens at the moment a drive dies. Neither decision is reversible without rebuilding the array, so it's worth getting right at order time. This guide explains the four levels that matter in practice (0, 1, 5, and 10) plus ZFS's take on the same ideas, with honest guidance on rebuild risk, what RAID does not protect against, and which level fits which workload.