The RAID 5 versus RAID 6 decision almost never comes up in the abstract. It surfaces while sizing a specific array, when a buyer looks at the raw capacity purchased, subtracts what parity consumes, and has to decide how much redundancy the workload actually needs. Both levels stripe data with distributed parity, and both keep most of an array's raw space, so on a capacity spreadsheet they look close. The difference comes down to one number: RAID 5 dedicates a single drive's worth of parity and survives one failure, while RAID 6 dedicates two and survives two. That extra parity block is the whole conversation.
What makes the second parity worth its cost is the rebuild window. When a drive fails in a RAID 5 set, the array has no redundancy until the replacement is fully rebuilt — and rebuilding means reading every surviving drive in full to recompute the lost member. If a second drive fails, or an unrecoverable read error surfaces during that long, I/O-heavy pass, the data is gone. RAID 6 keeps a second parity block through a single-drive rebuild, so the array stays protected while it recovers. As individual drive capacities climb into the multi-terabyte range and rebuilds stretch to many hours, that protection is exactly the risk buyers are paying to remove.
At a glance
Side by side
| Factor | RAID 5 | RAID 6 |
|---|---|---|
| Minimum drives | 3 | 4 |
| Parity | Single distributed parity across the set | Dual distributed parity — two independent parity blocks per stripe |
| Usable capacity | (N-1) drives — one drive of parity | (N-2) drives — two drives of parity |
| Fault tolerance | Any 1 drive; array is exposed until the rebuild completes | Any 2 drives; stays protected through a single-drive rebuild |
| Write penalty | ~4 back-end I/Os per small write (data plus one parity) | ~6 back-end I/Os per small write (data plus two parities) |
| Read performance | Strong when healthy; drops in degraded mode as parity is reconstructed | Strong when healthy; degrades similarly but can absorb a further loss |
| Rebuild risk with large drives | Higher — no redundancy during a long rebuild; a URE or second failure means loss | Lower — the second parity covers the single-drive rebuild window |
| Typical fit | Capacity-oriented, read-heavy sets built from moderate-size drives | Capacity-oriented sets using large drives where rebuild exposure matters |
Choose RAID 5 when
- Usable capacity per dollar is the priority and the drives are moderate, not the largest capacities available
- The workload is predominantly reads — file shares, backups, media libraries, and sequential archival tiers
- Drive counts are modest and rebuild windows stay short enough that single-failure exposure is acceptable
- Write rates are low enough that the parity write penalty will not bottleneck the array
Choose RAID 6 when
- Drives are large enough that a RAID 5 rebuild would leave the array exposed for many hours
- The array must survive a second drive failure or a read error that surfaces during a rebuild
- Continuity matters more than reclaiming the one extra drive that dual parity consumes
- You want capacity efficiency close to RAID 5 but need protection all the way through the rebuild window
Bottom line
Neither level is universally better — they trade the same axis in opposite directions. RAID 5 spends one drive on parity and survives a single failure, keeping usable capacity high, which suits read-heavy, capacity-oriented sets built from moderate-size drives. RAID 6 spends a second drive and carries a heavier write penalty, buying the one thing RAID 5 cannot offer: protection through the rebuild window, so a second failure or an unrecoverable read error during recovery no longer means data loss. As drive capacities and rebuild times grow, that is why RAID 6 has become the common capacity-oriented default for large arrays. Size the choice to your drive sizes and rebuild-risk tolerance — and remember no RAID level replaces backups.
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FAQ
Common questions
- Why choose RAID 6 over RAID 5 for large drives?
- Rebuilding a RAID 5 array reads every surviving drive in full to recompute the lost member, and the array has no redundancy until that finishes. With multi-terabyte drives that pass takes many hours, and a second failure or an unrecoverable read error during it causes data loss. RAID 6 keeps a second parity block through the rebuild, so the array stays protected while it recovers — which is why it is favored as drive sizes climb.
- How much capacity does RAID 6 cost versus RAID 5?
- RAID 5 reserves the equivalent of one drive for parity, giving (N-1) drives of usable space; RAID 6 reserves two, giving (N-2). On a small array that difference is large in percentage terms, but on a wide array of many drives it is modest — often a price worth paying for the extra fault tolerance. Both are far more space-efficient than mirroring, which halves raw capacity outright.
- Is RAID 6 slower than RAID 5?
- For small random writes, yes. RAID 5 needs roughly four back-end operations per partial-stripe write; RAID 6 needs about six, because it updates two independent parity blocks instead of one. Full-stripe sequential writes largely avoid that cost on both levels, and reads perform similarly. For write-heavy, latency-sensitive workloads such as busy databases, many engineers step past both toward RAID 10 instead.
- Can RAID 6 survive two drives failing at once?
- Yes — that is its defining feature. RAID 6 stores two independent parity blocks per stripe, so it tolerates any two simultaneous drive failures and stays protected while rebuilding after one. RAID 5 tolerates exactly one failure and is exposed until its rebuild completes. Neither level is a substitute for backups, but RAID 6 removes the single most common way large RAID 5 arrays lose data during recovery.