RAID & Redundancy Calculator
Two 8TB drives in RAID 1 do not give you 16TB. Pick your disk count and size to see what each RAID level really gives you in usable space.
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Capacities use decimal TB (what drive labels show); the usable space your OS reports after formatting will be slightly lower. RAID protects against disk failure — it is not a backup. ZFS equivalents: mirrors ≈ RAID 1, RAIDZ1 ≈ RAID 5, RAIDZ2 ≈ RAID 6.
Which RAID level should you pick?
RAID 0 stripes data for full speed but any single failure loses everything — only worth it for scratch data. RAID 1 mirrors every disk, so you lose half the capacity but can lose all but one disk. RAID 5 loses one disk's worth of capacity to parity, RAID 6 loses two, and RAID 10 combines mirroring with striping for fast rebuilds at the cost of half the space.
For most home servers with four or fewer drives, mirrors (RAID 1 or a ZFS mirror pool) are the sensible default: simple, fast to rebuild, and easy to expand two disks at a time. And whichever level you pick, remember that RAID only protects against hardware failure — a deleted file or a sync mistake still needs the backup strategy linked below.