💽 RAID Explained

Modern infrastructure demands resilience, speed, and data integrity. Whether you’re managing a homelab, a Proxmox cluster, or a full-scale data center, RAID (Redundant Array of Independent Disks) is foundational.

In this blog, I break down what RAID is, how each level works, when to use them, and the trade-offs involved. We’ll also explore how RAID fits into virtualized environments like QEMU and how to use it responsibly even with modern filesystems like ZFS.

🔍 What is RAID?

RAID is a storage technology that combines multiple physical drives into one logical unit to improve performance, fault tolerance, or both. It acts as a layer between your operating system and physical disks.

Benefits:

• Redundancy: Protects against drive failures
• Speed: Some levels improve read/write speed
• Scalability: Adds flexibility in managing storage pools

🔢 Popular RAID Levels

Let’s go through the most used RAID levels, starting from basic striping to hybrid setups.

RAID 0: Striping

[ A1 ][ A2 ]    (Disk 1 and 2 store alternating chunks of data)
[ B1 ][ B2 ]

• Performance-focused: Data is striped across disks
• No redundancy: If one disk fails, everything is lost
• Usable capacity: 100% (sum of all disks)

✅ Fastest read/write speeds
❌ Zero fault tolerance

Use case: Temporary storage, caching, non-critical workloads

RAID 1: Mirroring

[ A ][ A ]    (Each disk contains the same data)
[ B ][ B ]

• Redundancy-focused: Data is mirrored across disks
• High fault tolerance: One disk can fail without data loss
• Usable capacity: 50% of total

✅ Great reliability
❌ 50% disk waste

Use case: OS drives, important config storage, logs

RAID 5: Striping with Parity

[ A1 ][ A2 ][ P1 ]
[ B1 ][ P2 ][ B2 ]
[ C1 ][ C2 ][ P3 ]

• Performance + redundancy: Stripes data and parity across disks
• Fault tolerance: Can survive 1 disk failure
• Usable capacity: N-1 (e.g., 3 drives = 2 usable)

✅ Efficient balance
❌ Slow writes due to parity

Use case: General-purpose storage, archives

RAID 6: Dual Parity

[ A1 ][ A2 ][ P1 ][ P2 ]

• Better fault tolerance: Survives up to 2 disk failures
• Usable capacity: N-2

✅ Safer than RAID 5
❌ Slower performance, more disk overhead

Use case: Large data arrays, low-IOPS systems

RAID 10 (1+0): Mirrored Stripe

[ A1 ][ A2 ]
[ B1 ][ B2 ]    RAID 0 over RAID 1 mirrors

• Combines speed and redundancy
• Requires minimum 4 disks
• Usable capacity: 50%

✅ High performance and fault tolerance
❌ Expensive, uses 50% of disk space

Use case: Databases, virtualization platforms

📊 RAID Comparison Table (Extended)

Detailed Breakdown:

1. RAID Level: The type of RAID setup (e.g., RAID 0, RAID 1, etc.).
2. Min. Disks: The minimum number of physical disks required to implement this RAID level.
3. Redundancy: Indicates whether the RAID setup can tolerate disk failures:
   • ✅ means it provides some redundancy.
   • ❌ means no redundancy, so data will be lost if a disk fails.
4. Performance (Read Speed): How fast the system can read data.
   • ✅ Very Fast: Very high read speeds.
   • ✅ Good Speed: Fast but not as fast as RAID 0 or RAID 10.
   • ⚠️ Slower: Slower due to the overhead of managing parity (RAID 5 and RAID 6).
5. Performance (Write Speed): How fast the system can write data.
   • ✅ Very Fast: Very high write speeds.
   • ✅ Fast: Fast but not as fast as RAID 0 or RAID 10.
   • ⚠️ Slower: Slower due to the overhead of managing parity in RAID 5 and RAID 6.
6. Usable Capacity: The percentage of your total disk space that is available for storing data:
   • 100% means you can use all the disk space.
   • 50% means half of your total disk space is used for redundancy (mirroring or parity).
   • (N-1)% or (N-2)% means that the number of usable disks is reduced due to the redundancy mechanisms (1 or 2 disks are used for parity).
7. Pros: The main advantages of each RAID level, such as speed or redundancy.
8. Cons: The main drawbacks of each RAID level, such as high costs or reduced capacity.
9. Best Use Cases: Where each RAID level is best applied based on your needs.

Key Points:

• RAID 0: Perfect for maximizing speed but offers no data protection. If you need fast storage for temporary files or cache and data loss is not a concern, RAID 0 is a good choice.

• RAID 1: Offers excellent redundancy by mirroring data across two disks. It’s ideal for critical systems (like OS drives or config storage) where uptime and reliability matter.

• RAID 5: Stripes data across disks with parity, offering a good balance of performance and redundancy. Suitable for environments with a large number of files or where high storage capacity is needed but still requires protection from disk failures.

• RAID 6: Similar to RAID 5 but with two disks’ worth of parity. This makes it even more fault-tolerant, ideal for large-scale data storage that can’t afford any data loss.

• RAID 10: A combination of RAID 1 and RAID 0. Provides both excellent redundancy and very fast speeds, but you lose 50% of your total disk capacity due to mirroring. Best for high-performance applications like databases or virtualized environments.

💡 RAID in Virtual Environments (e.g., QEMU)

Running RAID inside QEMU using QCOW2 disks is technically possible and often used in homelabs and simulations. But it’s important to understand the limitations:

• Not physically redundant: All QCOW2 files sit on the same host filesystem. A host failure can render all RAID replicas useless.
• Useful for testing: It helps simulate disk failure and RAID behavior.
• You’ll fill disk space quickly: For example, with 2×200GB QCOW2 disks in RAID 1, you’ll effectively be mirroring data—so disk usage doubles.

Diagram: RAID 1 in QEMU

Virtual Machine
 ├── /dev/sdX (QCOW2 File A - 200GB)
 ├── /dev/sdY (QCOW2 File B - 200GB)
 │
 └─ mdadm RAID 1
       └── /dev/md0 (200GB usable, mirrored)

Recommendation:

• Use RAID in QEMU only for experimentation or logical integrity.
• For real redundancy, implement RAID on the host using ZFS or hardware RAID controllers.

🔧 Software vs Hardware RAID

• Hardware RAID: Managed by a RAID controller, offering better performance and easier configuration. Great for larger setups.
• Software RAID: Handled by the OS, with flexibility but requires more CPU and memory resources. Great for homelabs or smaller systems.

🚀 Conclusion

Understanding RAID levels is essential for building resilient systems that can handle data efficiently and safely. Choose the right RAID level based on your needs—whether you prioritize speed, redundancy, or a balance between the two. Always test your configurations in a lab environment to understand how they perform under failure conditions. RAID continues to be an invaluable tool, and knowing when and how to use it will ensure your infrastructure is reliable and fast.