RAID 1 for NAS: How Mirroring Works and How Much Space You Get
RAID 1 mirrors data across two drives. One drive holds your data; the other is an identical copy. If one drive fails, the NAS continues operating from the surviving drive with no data loss.
See exactly how much usable storage your configuration delivers.
How RAID 1 Works
Every write operation is duplicated to both drives simultaneously. Reads can come from either drive (and in practice NAS controllers often read from both drives in parallel to improve read performance). The result is full data protection against a single drive failure, at the cost of using only one drive's worth of space regardless of how many drives are in the mirror.
Capacity Formula
Usable = smallest drive capacity in the mirror pair.
With two 4TB drives in RAID 1: 1 × 3.64 TiB = 3.64 TiB raw before system and filesystem overhead. That is 22.7% of the 16 TB you might have spread across the same two drives in JBOD.
RAID 1 extends to more than two drives. Some NAS brands support 3-way or 4-way mirrors, but two-drive mirroring is by far the most common configuration.
Performance
- Read performance: Can be faster than a single drive. Reads can be split between both mirrors
- Write performance: Limited by the slower drive. Both drives must confirm each write
- Rebuild time: Fast. After a drive failure, a replacement drive is simply copied from the surviving mirror. Typically hours rather than the 12-48 hour rebuilds of RAID 5/6 on large arrays
When to Choose RAID 1
- 2-bay NAS: RAID 1 is your only redundant option. RAID 5 requires three drives minimum.
- Critical small datasets: OS partitions, databases, important files where speed of recovery matters more than capacity
- Simple setups: No complex parity calculations, no write penalty for parity, straightforward recovery
Frequently Asked Questions
Is RAID 1 better than RAID 5 for a 2-bay NAS?
It is the only option for a 2-bay NAS if you want redundancy. RAID 5 needs at least 3 drives. For a 4-bay NAS, RAID 5 gives significantly more usable capacity (3 drives vs 1 drive per set) with the same single-drive fault tolerance.
Can I use drives of different sizes in RAID 1?
Yes, but usable capacity is limited to the smallest drive. A 4TB and 6TB drive in RAID 1 gives only 3.64 TiB usable. The capacity of the 4TB drive. The extra space on the 6TB drive is unused.