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Choose RAID 5 if you need the capacity efficiency and your storage system can tolerate the risk of losing the array after a second drive fails during a rebuild. Choose RAID 6 if keeping the array available through a second drive failure is worth giving up another drive-equivalent of capacity and accepting additional parity work. Both levels protect against drive failures, not every cause of data loss, so neither replaces an independent backup.
What is the difference between RAID 5 and RAID 6?
Both RAID levels stripe data across drives and distribute parity information among them. Parity is information the system can use to reconstruct missing data after a drive failure.
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- RAID 5 has one distributed parity set and tolerates one failed drive.
- RAID 6 has two distributed parity sets and tolerates up to two failed drives.
In practical terms, RAID 6 can retain protection if another member drive fails while the array is already degraded by a first failure. RAID 5 has no equivalent second-failure tolerance: Seagate warns that a second drive failure before a rebuild completes causes data loss. These failure limits apply to drive failures within the array; they are not guarantees against other hardware faults, accidental deletion, malware, or site-level disasters. Seagate’s RAID concepts documentation explains the failure tolerance and rebuild behavior.
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For equal-capacity drives, a useful planning estimate is:
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- RAID 5: (number of drives − 1) × capacity of each drive.
- RAID 6: (number of drives − 2) × capacity of each drive.
For example, Seagate calculates five 8 TB drives in RAID 5 as 8 TB × (5 − 1), or 32 TB. Under the same equal-drive estimate, five 8 TB drives in RAID 6 provide 24 TB before formatting and system-specific overhead. That example is arithmetic based on the stated formula, not a guarantee of what a particular NAS will report as usable space.
For mixed-size drives, Seagate’s RAID 5 formula uses the smallest drive’s capacity multiplied by the number of drives minus one. Its example of four 16 TB drives and one 24 TB drive therefore yields 16 TB × (5 − 1), or 64 TB. Do not assume every controller supports mixed-size members or applies the same rules; check the device documentation for the exact layout and estimate. Seagate’s RAID-level guidance provides these formulas and examples.
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- (1) 1GB = 1 billion bytes and 1TB = 1 trillion bytes. Actual user capacity may be less depending on operating environment.
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Usable formatted capacity can be lower than the simple estimate, and manufacturers may express drive capacity differently from operating systems. Use the storage system’s own capacity calculator or documentation before creating an array.
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How do performance and rebuilds compare?
RAID 6 calculates an additional parity set, which adds parity work. Synology describes RAID 6 performance as lower than RAID 5, while Seagate says RAID 6 rebuilds more slowly because of its added parity calculations. Those statements do not establish a universal speed difference: results depend on the implementation, workload, hardware, and whether the array is operating normally or rebuilding. Synology’s RAID guidance applies to its storage systems.
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A rebuild restores redundancy after a failed drive is replaced; it is not a backup or a guarantee that the array will remain available. Rebuild duration depends on the drives, array size, controller, and operating conditions, so there is no responsible single time estimate for all systems. Seagate also notes that array performance may be affected during a rebuild. Until the rebuild finishes, RAID 5 is exposed to a second drive failure; RAID 6 can tolerate up to two failed drives, but the system’s current state and documentation matter.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Which configuration should you choose?
| Consideration | RAID 5 | RAID 6 |
|---|---|---|
| Drive failures tolerated | One | Up to two |
| Capacity overhead with equal-size drives | One drive-equivalent | Two drive-equivalents |
| Parity work | One distributed parity set | Two distributed parity sets |
| Rebuild comparison | Seagate says RAID 6 rebuilds more slowly because of its added parity calculations. | Seagate says rebuilds are slower than RAID 5 because of the added parity calculations. |
| Supported drive counts and layouts | Varies by storage product; check its documentation. | Varies by storage product; check its documentation. |
RAID 5 may fit when capacity is the priority
Consider RAID 5 when the extra usable capacity matters, the data and service can tolerate the one-drive-failure limit, and you have an independent backup and a plan to respond promptly to a failed drive. Its lower parity overhead does not establish a particular performance advantage for every workload.
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RAID 6 may fit when a second drive failure is a major concern
Consider RAID 6 when avoiding loss of the array after a second drive failure is more important than keeping the extra drive-equivalent of capacity. The added fault tolerance is especially relevant when rebuild exposure or the consequences of losing access are significant. It still does not eliminate the need for backups or product-specific validation.
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What to check before creating or changing an array
- Confirm support and minimum drive count. RAID levels, minimum disk counts, mixed-drive behavior, and other supported configurations vary by NAS or controller. Check the exact model’s current documentation and drive compatibility list; Synology’s guidance, for example, covers RAID types supported by its storage systems.
- Estimate usable capacity for your actual drives. Apply the vendor’s formula to the intended drive count and capacities, then verify the result with the storage system’s own calculator or documentation.
- Plan for a failure and rebuild. Know how the system reports a failed member, how to replace it, and what operational impact a rebuild may have. Do not count on an assumed rebuild time.
- Keep an independent backup. RAID redundancy does not protect against every way data can be lost. Seagate advises backing up important files before changing an existing array; verify that the backup is current and accessible before making changes.
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