RAID Levels Explained: Complete Guide to Storage Redundancy
What is RAID?
Redundant Array of Independent Disks (RAID) is a data storage virtualization technology that combines multiple physical disk drives into one or more logical units. Originally developed in 1987 at UC Berkeley, RAID provides either performance improvements, data redundancy, or both—depending on the configuration.
Why RAID Matters
Hard drives fail. It's not a question of "if," but "when." RAID is your first line of defense against data loss, but it is NOT a backup. Always maintain separate backups.
RAID 0: Striping (Performance)
RAID 0 splits data evenly across two or more disks (striping) with no parity or mirroring. This provides maximum read/write performance because all drives work simultaneously.
How It Works
When you write a 1GB file to a RAID 0 array with 4 drives, the file is split into 256MB chunks, and each chunk is written to a different drive in parallel. This means you get 4x the sequential write speed of a single drive.
⚠️ Critical Warning
RAID 0 offers zero redundancy. If ANY drive in the array fails, ALL data is permanently lost. Use this only for scratch disks, game libraries, or temporary video editing storage—never for irreplaceable data.
Best Use Cases
- Video editing scratch disks (footage is backed up elsewhere)
- Gaming installations (can be re-downloaded)
- High-performance computing temporary storage
RAID 1: Mirroring (Redundancy)
RAID 1 creates an exact copy (mirror) of data on two or more drives. If one drive fails, the system continues operating seamlessly from the surviving drive.
How It Works
Every write operation is performed on all drives simultaneously. A 1TB file written to a 2-drive RAID 1 array is written in full to both drives, consuming 2TB of raw storage for 1TB of usable space.
Performance Characteristics
- Read Speed: Can be faster than a single disk (some controllers read from both drives in parallel).
- Write Speed: Similar to a single disk (both drives must complete the write).
Best Use Cases
- Operating system boot drives
- Critical databases (SQL Server, PostgreSQL)
- Small business file servers
RAID 5: Distributed Parity (Balanced)
RAID 5 stripes data across all drives and stores parity information (a mathematical checksum that allows data reconstruction) distributed across all drives. This is the most popular RAID level for enterprise storage.
How Parity Works
Parity uses XOR (exclusive OR) logic. If Drive A has "01" and Drive B has "10", the parity is "11". If Drive B fails, the controller reconstructs Drive B's data by XORing Drive A ("01") with the Parity ("11") = "10".
⚠️ Rebuild Risk
During a rebuild after a drive failure, the array is in a "degraded" state with no redundancy. If a second drive fails during this period (which can be 12-48 hours for large drives), all data is lost. This is why RAID 6 was invented.
Capacity Calculation
Usable Capacity = (Number of Drives - 1) × Smallest Drive Size
Example: 4 × 4TB drives = (4-1) × 4TB = 12TB usable.
Best Use Cases
- File servers and NAS appliances
- Web servers
- Archive storage
RAID 6: Dual Parity (High Availability)
RAID 6 is identical to RAID 5 but stores two independent parity blocks per stripe, distributed across all drives. This allows the array to survive the simultaneous failure of any two drives.
When to Choose RAID 6 over RAID 5
- Using large-capacity drives (4TB+), where rebuild times are long.
- Mission-critical data where downtime is unacceptable.
- Arrays with more than 6 drives (higher statistical chance of concurrent failures).
RAID 10 (1+0): Nested Performance & Redundancy
RAID 10 combines the speed of RAID 0 (striping) with the redundancy of RAID 1 (mirroring). Data is first mirrored, then the mirrors are striped together. This is the gold standard for high-performance databases.
* Can tolerate at least one drive failure; potentially up to half of all drives if failures occur in different mirror pairs.
Best Use Cases
- Enterprise databases (SQL Server, Oracle, MySQL)
- High-transaction applications (e-commerce, banking)
- Virtualization host storage (VMware datastores)
Quick Comparison Table
| Level | Min. Drives | Fault Tolerance | Capacity (4x4TB) | Read Speed | Write Speed |
|---|---|---|---|---|---|
| RAID 0 | 2 | None | 16TB | Excellent | Excellent |
| RAID 1 | 2 | 1 drive | 8TB | Good | Average |
| RAID 5 | 3 | 1 drive | 12TB | Good | Slower (parity) |
| RAID 6 | 4 | 2 drives | 8TB | Good | Slowest (2x parity) |
| RAID 10 | 4 | 1+ drives | 8TB | Excellent | Excellent |
Hardware vs. Software RAID
Hardware RAID
Uses a dedicated RAID controller card with its own processor and memory. The OS sees a single logical disk.
- Pros: Faster, OS-independent, battery backup.
- Cons: Expensive, controller-specific (vendor lock-in).
- Examples: HPE Smart Array, Dell PERC, Broadcom MegaRAID.
Software RAID
Uses the host CPU and operating system to manage RAID. No specialized hardware needed.
- Pros: Free, flexible, portable across hardware.
- Cons: Uses CPU cycles, no protection before OS boot.
- Examples: Windows Storage Spaces, Linux mdadm, ZFS.
Final Recommendations
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Home User / NAS: RAID 5 or RAID 1. Prioritize redundancy over speed.
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Gaming PC: RAID 0 if you want speed, but understand the risk. SSDs are often fast enough without RAID.
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Small Business Server: RAID 6 or RAID 10. Don't risk business data on single-parity RAID 5.
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Enterprise / Database: RAID 10 for transaction-heavy workloads. Hardware controller strongly recommended.
Remember: RAID is NOT a Backup!
RAID protects against hardware failure. It does NOT protect against accidental deletion, ransomware, fire, or theft. Always maintain a 3-2-1 backup strategy: 3 copies of data, on 2 different media types, with 1 copy offsite.
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