What are RAID Levels and How Do They Differ?
Learn how RAID 0, 1, 5, and 6 differ in speed, capacity, and fault tolerance, with parity examples and OS interview questions answered.
Expected Interview Answer
RAID (Redundant Array of Independent Disks) combines multiple physical disks into one logical unit using striping, mirroring, or parity, and different RAID levels trade off capacity efficiency, performance, and fault tolerance in distinct ways — RAID 0 stripes for speed with no redundancy, RAID 1 mirrors for full redundancy, RAID 5 stripes with distributed parity to survive one disk failure, and RAID 6 adds a second parity block to survive two.
RAID 0 splits data into stripes written across all disks in parallel, maximizing throughput and usable capacity but offering zero fault tolerance — a single disk failure loses all data. RAID 1 mirrors every write to a second disk, so any one disk can fail with zero data loss, at the cost of halving usable capacity. RAID 5 stripes data across at least three disks and computes a parity block for each stripe, distributing parity round-robin across disks so any single disk failure can be reconstructed from the remaining disks and parity, giving good capacity efficiency (N-1 disks usable) with tolerance for exactly one failure. RAID 6 extends RAID 5 with a second, independently computed parity block, tolerating two simultaneous disk failures at the cost of two disks’ worth of capacity, which matters increasingly as disk rebuild times grow with larger drives, since a second failure during a RAID 5 rebuild would otherwise be catastrophic. Nested levels like RAID 10 (mirror then stripe) combine the speed of striping with the safety of mirroring at a higher hardware cost.
- RAID 0: maximum throughput and capacity, no redundancy
- RAID 1: full mirroring, survives one disk loss, simplest rebuild
- RAID 5: single-parity fault tolerance with efficient capacity use
- RAID 6: dual-parity fault tolerance, safer during long rebuilds
AI Mentor Explanation
RAID is like different ways a team distributes overs among bowlers. RAID 0 is splitting all overs evenly across bowlers purely for speed, with no backup plan if one gets injured — the innings collapses. RAID 1 is having a strict twin bowler mirror every over bowled by the first, so if one is injured the other seamlessly continues with identical figures. RAID 5 is spreading overs across several bowlers while keeping a rotating reserve bowler’s notes that can reconstruct any one bowler’s missing figures if they get injured. RAID 6 keeps two independent reserve bowlers’ notes, so the team survives even two simultaneous injuries.
Step-by-Step Explanation
Step 1
RAID 0 — striping
Data is split into stripes written in parallel across all disks for maximum speed and capacity, with no redundancy.
Step 2
RAID 1 — mirroring
Every write is duplicated to a second disk, so a single disk failure loses no data at the cost of half usable capacity.
Step 3
RAID 5 — striping with distributed parity
Data and rotating parity blocks are striped across disks, tolerating one disk failure while keeping N-1 disks of usable capacity.
Step 4
RAID 6 — dual distributed parity
A second, independent parity block is added per stripe, tolerating two simultaneous disk failures at the cost of two disks of capacity.
What Interviewer Expects
- Correct tradeoffs for RAID 0, 1, 5, and 6 in speed, capacity, and fault tolerance
- Understanding of how parity reconstruction works in RAID 5/6
- Awareness of why RAID 6 matters more as disks and rebuild windows grow larger
- Ability to recommend a RAID level for a given workload requirement
Common Mistakes
- Calling RAID 0 a backup solution when it has zero redundancy
- Confusing RAID 1’s mirroring with RAID 5’s parity mechanism
- Forgetting RAID 5 can only survive exactly one disk failure, not two
- Not knowing why RAID 6 or RAID 10 is preferred for large, slow-rebuilding drives
Best Answer (HR Friendly)
“RAID is a way of combining several physical disks so they act like one, and different RAID levels balance speed, storage efficiency, and safety differently. RAID 0 is fast but has no backup, RAID 1 fully duplicates data for safety, and RAID 5 and RAID 6 use clever parity math to survive one or two disk failures respectively while still keeping most of the capacity usable.”
Code Example
#include <stdio.h>
#define N_DISKS 4 /* 3 data blocks + 1 parity block per stripe */
/* Compute parity as XOR of the data blocks in a stripe */
unsigned char compute_parity(unsigned char data[N_DISKS - 1]) {
unsigned char parity = 0;
for (int i = 0; i < N_DISKS - 1; i++) {
parity ^= data[i];
}
return parity;
}
/* Reconstruct a missing block given the surviving blocks and parity */
unsigned char reconstruct_block(unsigned char surviving[N_DISKS - 2],
unsigned char parity) {
unsigned char rebuilt = parity;
for (int i = 0; i < N_DISKS - 2; i++) {
rebuilt ^= surviving[i];
}
return rebuilt; /* XOR of survivors + parity recovers the missing block */
}Follow-up Questions
- Why can RAID 5 only tolerate exactly one disk failure?
- How does RAID 10 combine mirroring and striping, and when would you choose it?
- What happens to RAID 5 performance during a rebuild after a disk failure?
- Why does RAID 6 become more important as individual disk capacity grows?
MCQ Practice
1. Which RAID level offers no redundancy at all?
RAID 0 only stripes data for performance and capacity; a single disk failure loses all data with no way to recover it.
2. How many simultaneous disk failures can RAID 6 tolerate?
RAID 6 keeps two independent parity blocks per stripe, allowing it to reconstruct data even if two disks fail at once.
3. What usable capacity does a RAID 5 array with N disks provide?
RAID 5 dedicates the equivalent of one disk’s worth of space to distributed parity, leaving N-1 disks of usable capacity.
Flash Cards
What does RAID 0 provide? — Striping for speed and capacity, with zero fault tolerance.
What does RAID 1 provide? — Full mirroring — any one disk can fail with zero data loss, at half capacity.
What does RAID 5 provide? — Striping with one distributed parity block per stripe, tolerating a single disk failure.
What does RAID 6 provide over RAID 5? — A second independent parity block, tolerating two simultaneous disk failures.