What is the main difference between RAID 5 and RAID 6?

RAID 5 uses one parity drive and survives one drive failure. RAID 6 uses two parity drives and survives two simultaneous failures. RAID 6 costs one more drive of capacity but is significantly safer for larger setups.

When is RAID 6 mandatory?

From 5 drives upward, or drive capacities of 12 TB and above. Rebuild times become long enough that URE risk under RAID 5 is statistically too high. Also recommended for critical data without separate backup.

Can I convert RAID 5 to RAID 6?

On mdadm, Synology and QNAP yes, online and without data loss – takes 12-30 hours per drive. ZFS RAIDZ doesn't support direct conversion; you have to recreate the pool and copy data over.

How much slower is RAID 6 than RAID 5?

About 5-10% slower on writes due to the second parity calculation. Reads are practically identical. In typical NAS workloads (media server, backup, file share) you won't notice the difference.

Is RAID 5 still safe with modern large drives?

With 4-8 TB drives and 3-4 bays: yes, acceptable. With 12 TB+ drives or 5+ drives: no, URE risk during a multi-hour rebuild becomes statistically too high. Then RAID 6 or RAIDZ2.

RAID 5 vs RAID 6: Which Should You Actually Pick?

Updated: 2026-04-29

RAID 5 or RAID 6? Most NAS builders make this call once – and only feel the consequences years later when the first drive dies. Here's the honest answer with real numbers instead of vendor marketing.

Short version

RAID 5 sacrifices one drive for parity, survives one drive failure. RAID 6 sacrifices two drives, survives two simultaneous failures. With 3-4 drives or capacities under 8 TB per drive: RAID 5 is fine. With 5+ drives or drives 12 TB and up: go RAID 6.

What each one actually does

Both distribute data plus parity information across all drives. If one drive fails, the system reconstructs the missing data from the remaining drives plus parity. RAID 5 calculates one parity, RAID 6 two – using different algorithms. That's where the extra safety comes from.

Capacity: what's left usable?

Rule of thumb:

RAID 5: (drive count − 1) × drive size

RAID 6: (drive count − 2) × drive size

Concrete examples with 16 TB drives:

4 drives: RAID 5 = 48 TB, RAID 6 = 32 TB. Difference: 16 TB (33% less under RAID 6)
6 drives: RAID 5 = 80 TB, RAID 6 = 64 TB. Difference: 16 TB (20%)
8 drives: RAID 5 = 112 TB, RAID 6 = 96 TB. Difference: 16 TB (14%)
12 drives: RAID 5 = 176 TB, RAID 6 = 160 TB. Difference: 16 TB (9%)

The more drives, the smaller the percentage capacity loss – and the more RAID 6 makes sense.

Reliability: rebuild is the real problem

A rebuild on 16 TB takes 12-20 hours, on 22 TB already 18-30 hours. During that time:

All remaining drives are under sustained read load (IOPS at the limit)
RAID 5 leaves you completely unprotected
A single read error (URE) on any remaining drive can abort the rebuild

Consumer drives have a specified URE rate of ~1 in 10^14 bits read. Sounds like a lot. But a rebuild of 4×16 TB reads 48 TB = 3.8×10^14 bits. Statistically: roughly 3× URE probability per rebuild. In practice that means: rebuild failure on large RAID 5 isn't doomsaying, it's a realistic risk.

RAID 6 still tolerates a second drive failure during rebuild OR a URE on another drive. That's exactly the difference.

Performance: write penalty

On writes both modes recalculate parity – pushing sequential writes to about 60-80% of raw drive speed (RAID 5) or 50-70% (RAID 6). Random writes suffer more. In typical NAS workloads (media server, backup target, file share): irrelevant. For databases or VMs: noticeable.

Cost: what the second parity costs

With 16 TB drives at ~$280:

4×16 TB: RAID 5 = $1120 for 48 TB = $23/TB. RAID 6 = $1120 for 32 TB = $35/TB. Premium: 50%.
6×16 TB: RAID 5 = $1680 for 80 TB = $21/TB. RAID 6 = $1680 for 64 TB = $26/TB. Premium: 25%.
8×16 TB: RAID 5 = $2240 for 112 TB = $20/TB. RAID 6 = $2240 for 96 TB = $23/TB. Premium: 17%.

Beyond 6 drives, RAID 6 is a no-brainer. With 4 drives the question is: 50% premium for second parity – or roulette?

When RAID 5 is still OK

3 drives – RAID 6 would leave only 1 usable drive, pointless
4 drives with ≤8 TB per drive – short rebuild times, low URE risk
Backup NAS that's itself mirrored from the main NAS
Pure media archive with additional cloud backup

When RAID 6 is mandatory

5 or more drives in one array
Drives 12 TB and up
Data without separate backup
Setups with same drive batch / same age (failures cluster in time)

Migration RAID 5 → RAID 6

Possible but uncomfortable. Synology, QNAP and mdadm can migrate online – 12-30 hours per drive, not interruptible, performance dismal during. ZFS doesn't support RAIDZ conversion; you have to recreate the pool and copy data over.

Lesson: plan RAID 6 from day one if drive count or capacity warrants it. Migration works but costs a day and your patience.

Alternatives

RAID 10 – faster random writes and higher IOPS, but 50% capacity goes to mirroring. Useful for VMs and DBs, rare in home use.

RAIDZ2 – ZFS variant of RAID 6 with block checksums against bitrot. More CPU, more RAM, much safer long-term.

SHR-2 – Synology variant, additionally allows mixed drive sizes. Synology hardware only.

For your specific setup, compare capacity and cost of both variants directly in the RAID Calculator.

RAID Complete Guide

What happens when RAID fails?

SMR vs CMR – why it matters in RAID

Best NAS Drives 2026

RAID Calculator