RAID 0: Striping Only
RAID 0 splits data in blocks across all drives in the array. A 4-byte file might store byte 1 on drive 1, byte 2 on drive 2, byte 3 on drive 3, and byte 4 on drive 4 — all written simultaneously. This parallelism makes RAID 0 the fastest RAID level for sequential reads and writes. Four drives in RAID 0 can theoretically deliver four times the throughput of a single drive.
The cost: zero redundancy. If any single drive fails, the entire array is lost. Not because of a design flaw, but because each drive holds only a portion of every file — without all drives, the data cannot be reconstructed. RAID 0 is appropriate only for expendable performance storage: video editing scratch space, render caches, test environments. Never for data you care about.
RAID 1: Mirroring
RAID 1 keeps identical copies of all data on two (or more) drives. Every write goes to both drives simultaneously. Every read can come from either drive — some implementations alternate reads between drives for faster read throughput. If one drive fails, the other continues serving data without interruption. Rebuilding after a failure copies the surviving drive onto a new replacement drive.
RAID 1 is simple, reliable, and recovers quickly from failure. The cost is capacity: two drives provide the usable space of one. A 2-bay NAS with two 4TB drives in RAID 1 gives 4TB of usable space. It is the standard for 2-bay home NAS devices where simplicity and reliability outweigh capacity efficiency.
RAID 5: Distributed Parity
RAID 5 distributes parity information across all drives. For every N drives, you get N-1 drives of usable storage. Parity blocks allow the array to reconstruct data from any single failed drive using XOR operations on the remaining drives and their parity. Reads are fast (parallel across all drives); writes require calculating and writing parity on every operation, making write performance slower than RAID 0 or RAID 1.
RAID 5 requires a minimum of 3 drives. With 4 drives (each 4TB), you get 12TB of usable space with 1-drive fault tolerance. The rebuild process after a drive failure stresses all remaining drives — if a second drive fails during rebuild (not uncommon with large drives), all data is lost. For this reason, RAID 5 is increasingly replaced by RAID 6 for arrays with drives larger than 2–4TB.
RAID 6: Dual Parity
RAID 6 adds a second independent parity block, allowing the array to survive two simultaneous drive failures. Minimum 4 drives; usable space is N-2 drives. A 6-drive RAID 6 (each 4TB) gives 16TB of usable space — the same as a 4-drive RAID 5 with 6 drives, but with the ability to survive two concurrent failures during normal operation or rebuild.
RAID 6 is the recommended level for larger arrays where rebuild times expose a meaningful risk window of a second failure. ZFS RAIDZ2 is the software RAID equivalent with better data integrity guarantees (bit rot detection and automatic repair) than traditional RAID 6.
RAID 10: Mirrored Stripes
RAID 10 (also called RAID 1+0) creates mirror pairs first, then stripes across the pairs. A minimum of 4 drives creates 2 mirror pairs; data is striped across those pairs. This delivers the read/write performance of striping (RAID 0 speed) with the redundancy of mirroring (RAID 1 safety). Any single drive in each mirror pair can fail without data loss. In a worst case, one full mirror pair can fail while the other survives.
RAID 10 uses 50% of raw capacity (same as RAID 1) regardless of drive count. It has the fastest write performance of any redundant RAID level because writes go directly to mirrored drives without parity calculation. Rebuild is faster than RAID 5/6 because it only copies one mirror to the replacement drive instead of recalculating parity across all drives. The cost is capacity efficiency — RAID 10 uses more drives for the same usable space than RAID 5/6.
All RAID Levels Comparison
| Level | Min Drives | Drive Failures Tolerated | Usable Capacity | Read | Write | Rebuild Speed |
|---|---|---|---|---|---|---|
| RAID 0 | 2 | 0 | 100% of all drives | Very fast | Very fast | N/A (total loss) |
| RAID 1 | 2 | N-1 (all but one) | 1 drive equivalent | Good (2x read) | Same as 1 drive | Fast |
| RAID 5 | 3 | 1 | (N-1) drives | Fast (N-1 parallel) | Medium (parity calc) | Slow (full parity rebuild) |
| RAID 6 | 4 | 2 | (N-2) drives | Fast | Slower (dual parity) | Slow |
| RAID 10 | 4 | 1 per mirror pair | 50% of all drives | Very fast | Very fast | Fast (mirror copy only) |
| RAIDZ1 (ZFS) | 3 | 1 | (N-1) drives | Fast | Medium | Slow |
| RAIDZ2 (ZFS) | 4 | 2 | (N-2) drives | Fast | Medium | Slow |
Frequently Asked Questions
Which RAID level is best for a home NAS?
For a 2-bay NAS: RAID 1 (simple, survives one drive failure). For a 4-bay NAS: RAID 5 or RAIDZ1 if capacity matters; RAID 6 or RAIDZ2 for better protection. For 6+ bays: RAIDZ2 or RAID 6 strongly recommended because rebuild windows on large drives are long enough that a second failure during rebuild is a real risk.
What happens when a RAID 5 drive fails?
The array enters degraded mode and continues operating using parity to reconstruct the missing data on the fly. Replace the failed drive as soon as possible. The array rebuilds automatically onto the new drive. During rebuild, performance is reduced and all remaining drives are under stress. A second failure during rebuild causes total data loss in RAID 5. Alerts and quick replacement are essential.
Is RAID 10 better than RAID 5?
For write-intensive workloads (databases, VMs), RAID 10 is significantly better — no parity calculation overhead means full write speed. RAID 10 also rebuilds faster. RAID 5 provides better capacity efficiency. For a home NAS with mostly sequential reads (media streaming), RAID 5 gives more usable capacity per drive and the performance difference is imperceptible.
What is Synology SHR (Synology Hybrid RAID)?
SHR (Synology Hybrid RAID) is Synology's proprietary RAID implementation. It provides RAID 1-equivalent protection for 2-drive setups and RAID 5-equivalent for 3+ drives, but also supports mixing different drive sizes efficiently — unused capacity from different-sized drives is utilized rather than wasted. SHR-2 provides dual-drive fault tolerance equivalent to RAID 6. SHR is the recommended RAID type for Synology NAS devices with mixed drive sizes.