RAID and how many drives you need
A clear explainer of RAID levels for a NAS - what each one costs you in drives, how much protection it gives, and how many disks to buy.
RAID spreads your data across several drives so it survives a disk dying, and usually reads and writes faster while it's at it. What it is not is a backup: it protects against a drive failing, not against a fat-fingered delete, ransomware, or a flood. Keep that distinction and the rest is just picking a level and a drive count.
The common RAID levels
RAID 1 - mirror (2 drives)
Two drives, identical copies. Survives one failure, gives you the capacity of one drive. It's the obvious choice for a 2-bay box.
RAID 5 / SHR - single parity (3+ drives)
Data plus one drive's worth of parity, spread across every disk. Survives one failure; usable capacity is your total minus one drive. The sweet spot for 3-5 mid-size drives.
RAID 6 / SHR-2 - double parity (4+ drives)
RAID 5 that can lose two drives at once. Usable capacity is total minus two drives. Once your disks are large or numerous, this stops being optional - see why below.
RAID 10 - striped mirrors (4+ drives, even count)
Mirrored pairs, striped for speed. Fast and tough, but you give up half your raw capacity. Worth it for VMs and databases; overkill for a media box.
How many drives should I buy?
| Goal | Level | Minimum drives | You "lose" |
|---|---|---|---|
| Simplest redundancy | RAID 1 | 2 | 1 drive |
| Space-efficient protection | RAID 5 / SHR | 3 | 1 drive |
| Protection for big arrays | RAID 6 / SHR-2 | 4 | 2 drives |
| Performance + redundancy | RAID 10 | 4 | half |
For most home builds, start at four bays in RAID 5 / SHR: three drives of usable space, one-drive protection. Step up to RAID 6 / SHR-2 once your drives pass ~12TB or the array grows beyond five or six disks.
Why drive count interacts with drive size
Bigger drives mean longer rebuilds. Resilvering a 16TB+ disk can run a full day or more, and for that entire window a single-parity RAID 5 array has zero protection left - a second drive picking that moment to fail takes the whole pool with it. That single fact is the reason to move to double parity as your drives get large. The drives most likely to fail during a rebuild are the ones that have been spinning next to it, same age, same batch, same heat.
Don't forget the spare and the backup
- A hot spare sits idle until a drive dies, then auto-rebuilds onto itself, shrinking the dangerous window.
- RAID still isn't a backup. Three copies, two media, one off-site.
Not sure how much usable space a layout gives you? Run the numbers in the RAID capacity calculator.
Ready to pick drives? Make them CMR and NAS-rated - how to choose a NAS drive covers it - then browse the buying guides.