RAID levels explained: 0, 1, 5, 6, and 10

RAID combines several drives into one volume that's faster, more resilient, or both. The level you pick decides how much capacity you keep and how many drives can fail before you lose data.

The one-sentence answer

For most home NAS builds, RAID 5 (or its ZFS equivalent) is the sweet spot — one drive of parity, the rest usable. Move to RAID 6 once your drives are large (16 TB+) or your array has six or more bays, because the rebuild window gets long enough that a second failure becomes a real risk.

The levels, briefly

  • RAID 0 (stripe) — splits data across all drives for speed and keeps every TB, but zero redundancy: one drive dies, everything is gone. Avoid for anything you care about.
  • RAID 1 (mirror) — every drive holds an identical copy. You keep one drive's worth of capacity no matter how many you add. Simple and bulletproof for two-bay setups.
  • RAID 5 (single parity) — one drive's worth of space is spent on parity; any single drive can fail and rebuild. Needs at least three drives.
  • RAID 6 (double parity) — two drives' worth of parity, so two can fail at once. Needs at least four drives. The safer choice for large arrays.
  • RAID 10 (mirror + stripe) — pairs of mirrors striped together. Half your raw capacity, but fast and quick to rebuild. Popular where write performance matters.

Capacity is the hidden cost

Redundancy isn't free — you pay for it in drives. Four 16 TB drives give 64 TB raw, but only ~48 TB usable in RAID 5, ~32 TB in RAID 6, and ~32 TB in RAID 10. The bigger your array, the smaller that parity tax becomes as a percentage.

Work out the exact numbers for your build with our RAID usable capacity calculator, then check the real price with the cost-per-usable-TB calculator.

RAID is not a backup

RAID protects against drive failure, not against deletion, ransomware, or a dead controller — all of which hit every drive at once. Pair any array with a real backup strategy; see the 3-2-1 backup rule.

Pick drives that survive a rebuild

A rebuild hammers every drive for hours, so the drives themselves matter as much as the level. Use CMR drives rated for NAS workloads — our NAS-Safe picks already exclude rebuild-stalling SMR. See why in CMR vs SMR.

General guidance, not a guarantee. RAID 10's real fault tolerance depends on which drives fail; always confirm your controller's behaviour and keep an independent backup.