RAID Write Penalty by Level
The write penalty is how many disk operations one random write from the server costs the array. It is why parity RAID writes more slowly than mirroring on the same drives.
Penalty per level
- RAID 0: 1 disk I/O per write.
- RAID 1 and RAID 10: 2.
- RAID 5 and RAID 50: 4.
- RAID 6 and RAID 60: 6.
Identical drives; decimal TB as drives are sold, binary TiB as most operating systems show it; before filesystem overhead. Hot spares are extra drives and add no capacity.
Random-write cost by level
| Level | Disk I/Os per random write | Random-write IOPS vs RAID 10 |
|---|---|---|
| RAID 0 | 1 | 200% |
| RAID 1 | 2 | 100% |
| RAID 5 | 4 | 50% |
| RAID 6 | 6 | 33% |
| RAID 10 | 2 | 100% |
| RAID 50 | 4 | 50% |
| RAID 60 | 6 | 33% |
Same drives, 100% small random writes, no controller cache. Reads carry no penalty.
Where the penalty comes from
A mirror writes each block to two drives. A RAID 5 small write reads the old data and old parity, then writes the new data and new parity: four operations. RAID 6 updates two parity blocks: six operations.
The formula
Random-write IOPS = (drives × IOPS per drive) ÷ write penalty. For a mix of reads and writes: array IOPS = raw IOPS ÷ (read share + write share × penalty). Use your drives’ own IOPS figure from the datasheet.
What changes the picture
A controller with write-back cache absorbs bursts of writes. Large sequential writes that fill whole stripes avoid the read-modify-write cycle, so the penalty matters most for small random writes such as databases and virtual machines.
1.92TB drives for this array
In-stock 1.92TB drives from our catalogue, with the price for the 12 drives in the example above. Hot spares are extra.
- 005051748 - EMC 1.92TB SAS 12Gb/s 2.5-inch Solid State Drive $154.95 each · 12 drives $1,859.40
- HPE Q8F87B 1.92TB SATA 6Gb/s 2.5-inch Solid State Drive $349.47 each · 12 drives $4,193.64
- MTFDDAK1T9TDC-1AT1ZABYY Micron 5200ECO Series 1.92TB 2.5 inch SATA3 TCG Disabled $447.37 each · 12 drives $5,368.44
- MZ1LB1T9HALS-000FB - Samsung PM983 Series 1.92TB Triple-Level Cell PCI Express 3.0 x4 $461.84 each · 12 drives $5,542.08
- WPP9G - Dell 1.92Tb Pcie 4.0 2.5 inch Solid State Drive $661.05 each · 12 drives $7,932.60
- MZ-ILT960A - Samsung PM1643 Series 1.92TB Triple-Level Cell SAS 12Gb/s 2.5-inch Solid $212.63 each · 12 drives $2,551.56
- THNSN81Q92CSE - Toshiba 1.92TB Multi-Level Cell SATA 6Gb/s 2.5-inch Read Intensive Solid $412.63 each · 12 drives $4,951.56
- MZ-QLW1T90 - Samsung PM963 Series 1.92TB Multi-Level Cell PCI-Express 3.0x4 2.5-inch $465.26 each · 12 drives $5,583.12
Frequently asked questions
What is the write penalty of RAID 6?
Six disk I/Os per small random write: read old data and both parity blocks, then write new data and both parity blocks.
Why is RAID 10 faster than RAID 6 for databases?
Each random write costs 2 disk I/Os in RAID 10 against 6 in RAID 6, so the same drives deliver about three times the random-write IOPS.
Does the write penalty affect reads?
No. Reads come straight from the data blocks; the penalty applies to writes.
RAID calculators by level
- RAID calculator (all levels)
- RAID 0 calculator
- RAID 1 calculator
- RAID 5 calculator
- RAID 6 calculator
- RAID 10 calculator
- RAID 50 calculator
- RAID 60 calculator
RAID planning guides
- How Many Drives Can Fail in Each RAID Level?
- RAID Hot Spare: What It Does and How It Affects Capacity
- TB vs TiB: Why a 24TB Drive Shows 21.8 TiB
- Best RAID Level for 2, 3, 4, 5, 6 or 8 Drives
- RAID 50 vs RAID 60
- RAID 6 vs RAID 60
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Updated 2026-10-22.