RAID Performance Calculator
Estimate read and write IOPS for a RAID array based on disk count, per-disk IOPS, and RAID write penalty.
Inputs
Typical: ~150-200 for 7.2K SAS/SATA HDD, ~90000+ for NVMe SSD
Remainder of the workload is treated as writes
Blended IOPS (given read/write mix)
632
Max Read IOPS
1,200
Max Write IOPS
300
RAID Write Penalty
4
Step by step
Values used
Number of Disks = 8; Single Disk IOPS = 150; RAID Level = RAID 5; Read % of Workload = 70
Read IOPS
read_IOPS = disks × single_disk_IOPS
Write IOPS
write_IOPS = (disks × single_disk_IOPS) / write_penalty
Blended IOPS (given read/write mix)
= 632
Max Read IOPS
= 1,200
Max Write IOPS
= 300
RAID Write Penalty
= 4
How it works
RAID read performance scales linearly with disk count since reads can be striped across all drives, but write performance is reduced by a RAID-level-specific write penalty. Parity RAID (5, 6) pays a read-modify-write penalty to recalculate and rewrite parity blocks, while mirrored RAID (10) pays a smaller, fixed 2x penalty for writing to both members of each mirror pair. A workload's blended IOPS depends on its read/write mix, since writes consume disproportionately more back-end disk operations.
Formulas
Read IOPS
read_IOPS = disks × single_disk_IOPS
Write IOPS
write_IOPS = (disks × single_disk_IOPS) / write_penalty
- P_{write}
- 4 for RAID 5, 6 for RAID 6, 2 for RAID 10
Frequently Asked Questions
Why is the RAID 5 write penalty 4 and RAID 6 is 6?
RAID 5 requires 4 back-end I/Os per logical write: read old data, read old parity, write new data, write new parity. RAID 6 requires 6, since it maintains two independent parity blocks: read old data, read both old parities, write new data, write both new parities.
Why is RAID 10's write penalty only 2 despite also being redundant?
RAID 10 writes are simple duplication — each logical write becomes two physical writes (to the primary and its mirror) with no parity calculation or read-before-write step, making it far more write-efficient than parity RAID at the cost of using half the raw capacity for redundancy.
Does adding more disks always increase blended IOPS?
Yes for a fixed RAID level and workload mix, since both read and write IOPS scale with disk count — but the relative benefit is workload-dependent: read-heavy workloads gain IOPS almost linearly, while write-heavy workloads on parity RAID gain more slowly because each added disk's IOPS is still divided by the same write penalty.