RAID 5 for NAS: Usable Capacity, Formula, and When to Use It

How RAID 5 Works

RAID 5 stripes data and parity across all drives in the array. No single drive holds all the parity. Each drive takes turns holding parity for different stripes. This means reads can use all drives simultaneously (read throughput scales with drive count) and a single drive failure can be recovered by recalculating the missing data from the parity and remaining drives.

Capacity Formula

Usable = (n − 1) × smallest drive capacity

With identical drives, this is straightforward:

• 3 × 4TB in RAID 5: 2 × 3.64 TiB = 7.28 TiB raw
• 4 × 4TB in RAID 5: 3 × 3.64 TiB = 10.91 TiB raw
• 6 × 4TB in RAID 5: 5 × 3.64 TiB = 18.19 TiB raw

With mixed drives, RAID 5 is limited to the smallest drive × (n−1). A 2TB, 4TB, 4TB, 4TB array in RAID 5 gives only 3 × 1.82 TiB = 5.46 TiB. SHR would give 8.18 TiB from the same drives.

RAID 5 vs SHR (Synology)

With identical drives, RAID 5 and SHR deliver the same usable capacity. SHR's advantage only appears with mixed drive sizes. If all your drives match and you are on Synology DSM, either option gives the same result. Though SHR is more flexible for future expansion with different drive sizes.

RAID 5 Write Penalty

Every write in RAID 5 requires reading the existing data and parity, computing new parity, then writing both the data and parity. This "RAID 5 write penalty" means RAID 5 has slower random write performance than RAID 1 or RAID 10 for the same hardware. For NAS workloads (large sequential transfers, media serving, backups), this is rarely a bottleneck.

Minimum Drives and Expansion

RAID 5 requires a minimum of 3 drives. Most 4-bay NAS users run 4-drive RAID 5. Rebuilds after a drive failure take longer with more or larger drives. A 4-drive array with 16TB drives can take 24-48 hours to rebuild, during which remaining drives are under sustained load.

Frequently Asked Questions

How much space does RAID 5 use for parity?

One drive's worth of capacity, regardless of array size. A 4-drive RAID 5 loses 25% to parity (1 of 4 drives). A 6-drive array loses only 17% (1 of 6 drives). Larger arrays are more efficient with RAID 5.

Is RAID 5 safe for large drives?

The risk during rebuild increases with drive size. A 20TB RAID 5 rebuild reads every byte of every remaining drive, which can take days and stresses those drives. RAID 6 adds a second parity drive, allowing the array to survive a second failure during rebuild. For drives 10TB+ in 4-bay setups, RAID 6 is worth considering.