Zerdly

RAID IOPS Calculator

Estimate the real, front-end IOPS a RAID array delivers to an application once the write penalty is taken into account. Enter the per-disk IOPS, the disk count, your read/write mix and the RAID level, and the tool returns functional IOPS, the raw back-end IOPS and the write penalty used.

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How to use this calculator

Enter Values

Enter your values and press Calculate to see results.

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How to use this calculator

  1. Enter the IOPS a single disk can sustain (from its datasheet or a benchmark) and the number of disks in the array.
  2. Enter your read workload as a percentage (default 70) — the rest is assumed to be writes.
  3. Enter the RAID level as a number (0, 1, 5, 6 or 10) and read off the functional IOPS.

How it works

Raw back-end IOPS is per-disk IOPS times the number of disks. Reads pass straight through, but every host write becomes several back-end operations because the mirror or parity has to be updated too — this is the write penalty: 1 for RAID 0, 2 for RAID 1 and RAID 10, 4 for RAID 5 and 6 for RAID 6. Functional IOPS = raw × read% + raw × (1 − read%) ÷ write penalty. So a write-heavy workload on RAID 6 sees far fewer usable IOPS than the same disks in RAID 10.

Worked example

Worked example. Eight disks at 150 IOPS each in RAID 5 with a 70% read mix: raw = 150 × 8 = 1,200 IOPS. Reads give 1,200 × 0.70 = 840; writes give 1,200 × 0.30 ÷ 4 = 90. Functional IOPS = 840 + 90 = 930.

Common mistakes

  • Ignoring the write penalty and sizing an array on raw IOPS — a parity RAID can deliver a fraction of the raw figure under heavy writes.
  • Using an optimistic per-disk IOPS — a 7,200 rpm HDD does roughly 75–100 random IOPS, not the thousands an SSD manages; use a figure that matches your actual drives.
  • Forgetting that controller cache and large sequential I/O hide much of the penalty, so this estimate is worst-case-ish for small random writes, not a guarantee.

Frequently asked questions

Why is the RAID 5 write penalty 4?

A single small write in RAID 5 requires four disk operations: read the old data, read the old parity, write the new data, and write the new parity. RAID 6 keeps two independent parities, so it needs six operations and has a penalty of 6.

Does read caching change these numbers?

Yes. Controller and OS read caches, plus SSD tiers, can serve many reads without touching the disks, pushing real functional IOPS above this estimate. Conversely a cold cache or fully random workload will track closer to the calculated figure. Treat the result as a planning baseline.

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