Hardware RAID vs software RAID
Hardware RAID runs on a dedicated controller with its own processor and cache, offloading parity from the CPU and often adding battery-backed write cache. Software RAID (mdadm, ZFS, Storage Spaces) runs on the host CPU with no extra card and no vendor lock to a specific controller.
A hardware RAID controller presents a single logical volume to the OS and handles parity, rebuilds, and caching itself. With a battery or flash-backed write cache, it can safely acknowledge writes before they hit disk, which speeds parity RAID. The tradeoff is cost, and the fact that a failed controller may need a compatible replacement to read the array.
Software RAID moves that work to the host CPU. Modern CPUs handle parity easily, and ZFS adds checksums, snapshots, and self-healing that hardware RAID does not provide. The array is portable across machines because it is not tied to a proprietary controller. The cost is CPU cycles and, for safe write caching, careful use of a power-loss-protected SSD for the log device.
When buying, note that many servers ship with an HBA in IT mode (pass-through) rather than a RAID card, which is exactly what ZFS and mdadm want. If you need hardware RAID, confirm the exact controller model and whether the cache battery or capacitor is present and healthy, since refurb cards often arrive with dead batteries.
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