How to use the RAID Capacity and Fault Tolerance Calculator
- 1. Choose the levelPick a RAID level. For RAID 50 or 60, also enter the number of sub-arrays.
- 2. Enter the disksType the number of disks and the size of each. Add per-disk IOPS if you want the write-penalty estimate.
- 3. Read the resultSee usable capacity, efficiency, failures survived, write penalty and a stripe-by-stripe block layout.
How each RAID level stores data
- RAID 0 stripes blocks across all disks. You get full capacity and speed but no redundancy: one failure loses everything.
- RAID 1 mirrors the same data onto every disk. Usable space is one disk.
- RAID 5 stripes data with one parity block per stripe, rotated across disks. Usable space is (n − 1) disks and it survives one failure.
- RAID 6 adds a second parity block (P and Q). Usable space is (n − 2) disks and it survives any two failures.
- RAID 10 stripes across mirror pairs. Usable space is n ÷ 2. It always survives one failure, and up to one per pair.
Worked example
Four 4 TB disks in RAID 5: raw capacity = 4 × 4 = 16 TB, usable = (4 − 1) × 4 = 12 TB, efficiency 75%, one failure survived. The same disks in RAID 10 give 8 TB (50%). Six 2 TB disks in RAID 6 give (6 − 2) × 2 = 8 TB.
Write penalty matters for databases. A small random write in RAID 5 needs 4 I/Os: read old data, read old parity, write new data, write new parity. With four disks of 100 IOPS and a 50% read mix: 400 × 0.5 + 400 × 0.5 ÷ 4 = 200 + 50 = 250 usable IOPS.
Parity in one line
RAID 5 parity is the XOR of the data blocks in a stripe. If one disk fails, XOR of the surviving blocks rebuilds the missing one. While it rebuilds, a second failure loses the array, which is why large disks often use RAID 6.
Frequently asked questions
Is RAID a backup?
No. RAID protects against disk failure, not against deletion, ransomware or a controller fault. Keep separate backups.
Why is my usable space smaller than the result?
Drive makers count 1 TB as 10^12 bytes, while operating systems show TiB (2^40 bytes), about 9% less. File systems also reserve some space.
Can I mix disk sizes?
Most RAID controllers treat every disk as the size of the smallest one, so the extra space on bigger disks is unused.
Which RAID level is best for a database?
RAID 10 is common because its write penalty is only 2. RAID 6 is common for large, read-heavy storage.