RAID Levels Explained: RAID 0, 1, 5, 6 and 10 for NAS

RAID combines several drives so that the NAS keeps working when one of them fails. Each level trades usable space against protection.

The common RAID levels

• RAID 0: all drives joined for speed and space, with no protection. One failure loses everything. Not for business data.
• RAID 1: two drives that mirror each other. You get one drive of space and survive one failure. The standard choice for a 2-bay NAS.
• RAID 5: three or more drives. You lose one drive of space to protection and survive one failure. The usual choice for 4-bay and 5-bay units.
• RAID 6: four or more drives. You lose two drives of space and survive two failures. Recommended for six drives or more, and for large drives.
• RAID 10: four or more drives in mirrored pairs. You get half the total space, with fast writes and quick rebuilds. Used for databases and virtual machines.

Why large drives change the choice

When a drive fails, the NAS rebuilds its contents onto a replacement. With 16 TB or 20 TB drives a rebuild can run for a day or more, and every other drive is working hard throughout. In RAID 5 a second failure during that window loses the volume. RAID 6 removes that risk, which is why it is the safer default for bigger arrays.

Brand-specific options

Synology offers SHR, which behaves like RAID 5 or 6 but lets you mix drive sizes. QNAP's QuTS hero adds RAID-TP, which survives three failures. Asustor supports the standard levels. These are refinements of the same idea.

RAID is not backup

RAID protects against a drive failing. It does nothing if a file is deleted or encrypted, or if the whole NAS is lost. You still need separate copies with version history, one of them off-site.

Common questions