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RAID Usable Capacity

Usable storage, efficiency, and fault tolerance for RAID 0/1/5/6/10 given drive count and size.

InputRAID0: N·S , RAID1: 1·S , RAID5: (N−1)·S , RAID6: (N−2)·S , RAID10: (N/2)·S

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The engineering

Every RAID level trades raw drive capacity for either speed, redundancy, or both. RAID 0 stripes with zero protection; RAID 1 mirrors for full redundancy at 50% efficiency; RAID 5 and 6 spend one and two drives on distributed parity respectively; RAID 10 mirrors then stripes, giving good rebuild behavior at half capacity. This card assumes equal-size drives — the usual and recommended case.

The gotcha is rebuild risk on large arrays. A single-parity RAID 5 of big drives is exposed to a second failure during the hours-to-days rebuild window, which is why RAID 6 (dual parity) is standard for arrays of large-capacity disks. Also remember drive marketing uses decimal TB (10¹²), so formatted OS capacity in TiB will read ~9% lower.

Where this math comes from

The term RAID comes from a 1988 SIGMOD paper by David Patterson, Garth Gibson, and Randy Katz at UC Berkeley, titled "A Case for Redundant Arrays of Inexpensive Disks." Their pitch was economic: instead of one expensive mainframe-grade disk, gang together many cheap PC-class drives and use redundancy to beat the reliability penalty of having more moving parts. The paper laid out five levels, RAID 1 through 5, each with a different redundancy scheme.

Industry ran with it fast. Striping-only arrays got retroactively labeled RAID 0, dual-parity RAID 6 was added to survive two concurrent failures as drives grew, and nested schemes like RAID 10 combined mirroring and striping. The "Inexpensive" in the acronym quietly became "Independent" once RAID moved into enterprise storage that was anything but cheap.

  1. 1988Patterson, Gibson & Katz (UC Berkeley)Publish the RAID paper defining levels 1–5.
  2. 1988Industry conventionPlain striping named RAID 0 retroactively.
  3. 1993Storage vendorsRAID 6 dual-parity added for two-drive fault tolerance.
  4. 2009SNIA / DDF standardCommon RAID Disk Data Format standardizes layout across controllers.

See the full timeline of the math behind every calculator →

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