RAID Performance Calculator
Estimate combined read and write IOPS across a RAID array based on level and disk count.
Inputs
Percentage of I/O that is reads (remainder is writes)
Effective IOPS
632
Effective Read IOPS
442
Effective Write IOPS
189
Write Penalty Factor
4
Total Raw IOPS
1,200
Step by step
Total raw IOPS
n × disk_IOPS = 6 × 200
= 1200
Write penalty for RAID 5
4
= 4x
Effective IOPS
raw / (read% + write% × penalty) = 1200 / (0.7 + 0.3 × 4)
= 632
How it works
RAID write penalties reflect the extra I/O operations required per logical write: RAID 0 has no penalty (1:1), RAID 1/10 must write to both mirrors (2:1), RAID 5 must read old data + old parity + write new data + write new parity (4:1), and RAID 6 adds a second parity update (6:1). Effective IOPS is calculated by dividing the total raw IOPS by the weighted average of read and write penalties based on the workload's read/write ratio.
Formula
Effective IOPS
effective_IOPS = (n × disk_IOPS) / (read% + write% × write_penalty)
- n
- disk count
- I_{disk}
- IOPS per disk
- r
- read fraction
- w
- write fraction
- P_w
- write penalty factor
Frequently Asked Questions
Why does RAID 5 have a write penalty of 4?
Each logical write requires reading the old data stripe, reading the old parity stripe, writing the new data stripe, and writing the recalculated parity stripe — 4 physical I/O operations total.
How does read/write ratio affect performance?
Higher read percentages improve effective IOPS because reads have no penalty (1:1), while writes multiply the load by the penalty factor — a 90% read workload on RAID 5 performs much closer to raw disk IOPS than a 50/50 workload.