The RAID write penalty is the hidden tax that makes parity RAID slow for writes: each host write costs several back-end disk operations. Here's the maths — and see it in your IOPS in the RAID calculator.
What the write penalty is
Every host write to a redundant array triggers extra back-end I/O. For mirroring (RAID 1/10) the data is written to both copies — a ×2 penalty. For RAID 5, a small write must read the old data and old parity, then write the new data and new parity — four operations, a ×4 penalty. RAID 6 updates two parities, giving a ×6 penalty.
These factors are standard: RAID 0 ×1, RAID 1/10 ×2, RAID 5 ×4, RAID 6 ×6. They only affect writes; reads have no penalty.
How it hits IOPS
Front-end (host) write IOPS = total back-end drive IOPS ÷ write penalty. So an array of drives capable of 1,000 back-end IOPS delivers ~250 host write IOPS in RAID 5 (÷4) but ~500 in RAID 10 (÷2). For a mixed read/write workload the effective figure sits between the read and write numbers, weighted by the mix.
This is exactly why write-heavy transactional databases favour RAID 10 despite its 50% capacity cost — see RAID 6 vs RAID 10. The calculator computes read, write and mixed IOPS for any level and drive class.
The exceptions: WAFL and ZFS
Some systems sidestep the classic penalty. NetApp's WAFL (RAID-DP/TEC) writes full stripes to free space, avoiding the read-modify-write cycle — so it doesn't carry the ×6 tax. ZFS uses copy-on-write and variable stripes, so its write behaviour also differs (and a RAIDZ vdev's IOPS are limited to ~one drive instead). Our calculator models these correctly rather than applying a misleading ×6.
Flash also softens the impact: SSDs have so many IOPS to spare that the penalty matters less in absolute terms than on spinning disks — but it still applies.