RAID controllers
The controller is the part that decides whether your disks form a redundant array or stay independent volumes. You usually discover the wrong choice at the moment a disk fails and there is no way back.
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Buying guide
Hardware RAID or HBA?
If you run a conventional operating system (Windows Server, Linux on ext4 or XFS), a hardware RAID controller with cache and battery backup is the right answer: in-flight writes survive a power cut. If you run ZFS, Ceph or Storage Spaces, the opposite holds — they want to see raw disks, and a hardware RAID layer only obstructs them. There you need an HBA.
AROC and PCIe do not interchange
An AROC card mounts in the board’s dedicated slot and leaves your PCIe slots free; a PCIe card goes in an ordinary expansion slot. The two are not interchangeable, and each generation has its own AROC slot: the P440ar is Gen9 only, the P408i-a Gen10 only. Match your server generation against the compatibility list on the product before ordering.
Do not forget the cache battery
Without a battery or capacitor backup, a RAID controller disables write-back caching itself and falls back to write-through. The result is a marked drop in write speed with no error shown anywhere. Old batteries also lose capacity after a few years and cause the same thing; iLO reports the state under Cache Module.
Frequently asked questions
Does RAID replace backups?
No, and this is the most dangerous misconception in server operations. RAID protects only against a disk failing as hardware. Accidental deletion, ransomware, database corruption, fire and theft are none of them covered, because each replicates across every disk the instant it happens. RAID buys availability, not recovery.
Which RAID level should I pick?
For the OS volume, RAID 1 (two mirrored disks) is simple and dependable. For general storage, RAID 10 gives the best balance of speed and safety but costs half the capacity. RAID 5 yields more usable space, but with today’s large disks a rebuild takes hours, and a second failure in that window loses the whole array; above 4 TB per disk, RAID 6 is the safer choice.