RAID (Redundant Array of Independent Disks)

Overview

RAID (Redundant Array of Independent Disks) is a method of combining multiple physical drives into a single logical storage system to provide:

• Increased capacity
• Improved performance (in some configurations)
• Resilience against disk failure

RAID is widely used across:

• CCTV / video recording systems
• Servers and virtualisation hosts
• NAS / SAN storage systems

In CCTV systems in particular, RAID is primarily used to provide continued recording during disk failure, rather than performance.

Key Concept: RAID ≠ Backup

RAID provides availability, not data protection.

• RAID protects against hardware failure (disk loss)

• RAID does not protect against:

  • Data corruption
  • Accidental deletion
  • Malware / ransomware
  • System failure

A RAID array can be fully operational while the stored data is already lost or corrupted.

Common RAID Levels

RAID 0 - Striping (No Redundancy)

• Data is split across multiple drives
• Provides maximum capacity and performance
• No fault tolerance

Minimum drives: 2

Usable capacity: 100% of total

If one drive fails → all data is lost

Use case:

• Temporary / non-critical storage only
• Not suitable for CCTV or life safety systems

RAID 1 - Mirroring

• Identical data written to multiple drives
• Provides simple redundancy

Minimum drives: 2

Usable capacity: 50%

If one drive fails → system continues running

Use case:

• Small systems
• Simple NVRs
• OS drives

RAID 5 - Striping with Single Parity

• Data + parity distributed across drives
• Can tolerate one drive failure

Minimum drives: 3

Usable capacity: (N - 1)

Example:

• 4 × 8TB → 24TB usable

Failure behaviour:

• 1 drive fails → system runs degraded
• 2nd drive fails → total data loss

Key issue:

• Rebuild times on large disks are long
• High risk during rebuild window

Use case:

• Medium CCTV systems (legacy standard)
• Light server workloads

RAID 6 - Striping with Dual Parity

• Similar to RAID 5, but with two parity blocks
• Can tolerate two simultaneous drive failures

Minimum drives: 4

Usable capacity: (N - 2)

Example:

• 6 × 8TB → 32TB usable

Advantages:

• Much safer for large arrays
• Reduced risk during rebuild

Use case:

• Larger CCTV systems
• Higher availability requirements

RAID 10 - Mirrored Stripes

• Combines RAID 1 + RAID 0
• Data is mirrored, then striped

Minimum drives: 4 (even number required)

Usable capacity: 50%

Advantages:

• Fast rebuild times
• High performance
• Strong resilience

Use case:

• High-performance systems
• Critical recording environments

Usable Capacity vs Raw Capacity

RAID reduces usable storage depending on configuration.

CCTV storage calculations must always be based on usable capacity, not raw disk size.

Rebuild Behaviour & Risk

When a drive fails, the array must rebuild onto a replacement disk.

Key considerations:

• Rebuilds can take many hours or days on large drives

• During rebuild:

  • RAID 5 → no redundancy remaining
  • RAID 6 → reduced redundancy

• Additional disk failure during rebuild = data loss

Practical implication:

RAID 5 becomes increasingly risky with:

• Large drives (8TB+)
• Large arrays
• Continuous write systems (like CCTV)

RAID in CCTV Systems

RAID is commonly used in NVRs and VMS platforms to:

• Maintain recording during disk failure
• Provide predictable storage capacity
• Support continuous write workloads

Important characteristics of CCTV workloads:

• Constant sequential writes
• High disk utilisation
• Large storage volumes
• Long retention periods

Typical RAID Choices in CCTV

Common Mistakes

1. Assuming RAID = Backup

It is not. A failed array or corrupted system still results in data loss.

2. Using RAID 5 with Large Drives

Rebuild risk becomes significant with modern disk sizes.

3. Ignoring Rebuild Time

Systems may be vulnerable for extended periods after a failure.

4. Calculating Storage Using Raw Capacity

Always calculate based on usable capacity after RAID.

5. Mixing Drive Types

Different drive speeds / sizes can reduce performance and reliability.

Practical Guidance

• Prefer RAID 6 over RAID 5 for larger systems

• Use surveillance-rated drives for CCTV workloads

• Allow additional overhead for:

  • Metadata
  • Filesystem usage
  • System reserves

• Validate storage using a calculation tool:

See: CCTV Storage Calculator (FireSecure Tools)

Summary

RAID is a fundamental component of modern storage systems, providing:

• Increased availability
• Controlled failure tolerance
• Scalable capacity

However, it must be properly understood and applied:

• It does not replace backups
• It introduces capacity overhead
• It carries rebuild risk

Correct RAID selection is critical in CCTV systems to ensure reliable recording and retention under real-world conditions.