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RAID 4 calculator

Block striping with a dedicated parity disk — RAID 5 with parity on one drive. Set your drives below for live usable capacity, fault tolerance, IOPS, rebuild time and URE risk.

DataDistributed parity

1 · Choose a RAID level

Stripe & mirror
Single parity
Dual / triple parity
Nested
ZFS RAID-Z

Block striping with a dedicated parity disk.

2 · Configure drives

3 · Drive class

3.5" nearline SAS/SATA capacity HDD — indicative figures.

Advanced — read/write mix, URE rate
RAID 4 · 5 × 8 TB
32 TB usable
of 40 TB raw · 80% efficiency
Fault tolerance1 drive
Write penalty×4
IOPS estR ≈600 · W ≈150 · mix ≈316
Throughput estR ≈1K · W ≈1K MB/s
Rebuild / drive est≈ 27.8 h
URE on rebuild risk22.6%

During a single-drive rebuild there is no remaining redundancy — a URE on a surviving drive means data loss for the affected stripe. Real controllers mitigate via patrol reads/scrubs, so field results are often better.

Capacity distribution80% usableUsable: 32 TB32Parity: 8 TB8Usable · 32 TBParity · 8 TB
Fault tolerance — parity per arrayDDDDPDataParity1 drive
IOPS — back-end budget vs deliveredBack-end budget600Front-end read600Front-end write150Write penalty ×4 — each host write costs 4 back-end I/Os
URE risk during a single-drive rebuild0%25%50%75%100%23%data read during rebuild (64 TB →)URE 1 in 10^15

Calculated for planning. We don't publish prices — a 24-year UK reseller, Servnet confirms the exact drives, array and pricing on quote. IOPS, throughput & rebuild are indicative estimates.

Overview

What RAID 4 is

RAID 4 stripes data across drives and stores all parity on one dedicated disk. Usable capacity is (n−1) × drive size — identical to RAID 5 — and it survives a single drive failure. The difference from RAID 5 is that parity lives on a single drive rather than being distributed.

That dedicated parity disk becomes a write bottleneck (every write updates it), which is why general-purpose arrays use RAID 5 instead. RAID 4 persists mainly in specific appliances (notably some NetApp configurations) where the architecture mitigates the bottleneck.

At a glance
Usable capacity(n − 1) × drive size
Minimum drives3
Fault tolerance1 drive
Write penalty×4
Worked example
5 × 8 TB nearline HDD → 32 TB usable, survives 1

Five 8 TB drives in RAID 4 give 32 TB usable (80%), one drive of which is dedicated parity. Capacity matches RAID 5, but the single parity disk caps write performance — which is why RAID 5 replaced it for general use.

Advantages

  • Same capacity efficiency as RAID 5 — (n−1)/n
  • Survives one drive failure
  • Simple, predictable layout
  • Good sequential read performance

Trade-offs

  • Dedicated parity disk is a write bottleneck
  • ×4 write penalty
  • No redundancy during rebuild (single parity)
  • Largely superseded by RAID 5

Best for

  • Appliances designed around dedicated parity
  • Sequential, read-heavy workloads
  • Educational / legacy contexts

Consider another level when

  • General-purpose arrays (use RAID 5/6)
  • Write-heavy workloads
  • Large drives where rebuild risk matters
Level landscape — efficiency vs fault tolerance (typical)012325%50%75%100%drives survivedspace efficiency →RAID 0RAID 5RAID 50RAID-Z1RAID 6RAID 60RAID-Z2RAID-Z3RAID 10RAID 1

RAID 4 — common questions

How is RAID 4 capacity calculated?

Usable capacity is (number of drives − 1) × drive size — the same as RAID 5 — because one drive holds parity. Five 8 TB drives give 32 TB usable.

RAID 4 vs RAID 5?

Same capacity and fault tolerance, but RAID 4 keeps all parity on one dedicated disk (a write bottleneck) while RAID 5 distributes parity across all drives. RAID 5 is preferred for general use; RAID 4 survives mainly in specific appliances.

Is RAID 4 still used?

Rarely in general-purpose controllers, but it underpins some storage appliances whose architecture removes the dedicated-parity bottleneck. For DIY arrays, choose RAID 5 or RAID 6.