RAID 5 (Redundant Array of Independent Disks) is often chosen for its balance between performance, storage capacity, and fault tolerance. However, when used with a large number of disks, it can become a risky solution. This article explores the limitations of RAID 5, particularly when it includes more than four disks, and suggests alternatives like RAID 6, while explaining the underlying scientific principles.

Understanding How RAID 5 Works

RAID 5 distributes data across multiple disks and uses a parity block to ensure redundancy. If a disk fails, the data can be rebuilt using the parity information, providing fault tolerance without requiring a dedicated backup disk.

However, a major limitation is that RAID 5 only tolerates a single disk failure. If a second disk fails before the first is rebuilt, all data is lost.

The Limitations of RAID 5 with More Than Four Disks

As the number of disks increases, so do the risks. Here’s why:

Higher risk of multiple failures:

With a large number of disks, the likelihood of a second disk failing during the rebuild process increases significantly.

Long rebuild times:

The more disks there are, the slower the rebuild process. During this time, the RAID array remains vulnerable.

Increased stress on remaining disks:

The rebuild process places heavy stress on the functioning disks, which can accelerate their failure.

Statistical and Scientific Explanation

The risk of combined disk failures grows as the number of disks increases. In theory, a disk with a MTBF (Mean Time Between Failures) of one million hours seems reliable. However, in a RAID 5 with multiple disks, the probability of a second failure during rebuild becomes significant. This is why RAID 5 is not recommended for arrays with more than four disks.

A Real-World Example: Data Loss in RAID 5

The risk of data loss in RAID 5, especially in large configurations (8 to 16 disks), is very real. I experienced this firsthand: one disk failed, and during the rebuild process, a second disk also failed, resulting in total data loss. Fortunately, the server contained backups of a non-critical project, limiting the impact. In a desperate attempt to recover the data, I contacted DELL, the server manufacturer. Their technician confirmed that these scenarios are common in large RAID 5 arrays. Their advice was clear: use RAID 6 at a minimum to tolerate two simultaneous disk failures and avoid such disasters.

Safer Alternatives: RAID 6 and Other Solutions

RAID 6: Additional Protection

RAID 6 improves data security by using two parity blocks, allowing for the simultaneous failure of two disks.

Advantages:

• Tolerance for two disk failures.
• Significantly reduced risk of data loss during rebuild.

Disadvantages:

Other Alternatives

• RAID 10 (1+0): Combines the benefits of RAID 1 (mirroring) and RAID 0 (striping) for high performance and fault tolerance, but requires more disks for the same capacity.
• RAID 50 / RAID 60: Hybrids between RAID 5/6 and RAID 0, providing a good balance of performance and security.
• Synology SHR-2: A flexible technology that allows the combination of disks of different sizes with fault tolerance equivalent to RAID 6.
• Windows Storage Spaces and ZFS: Advanced solutions offering features like dual parity and automatic data repair, ideal for critical environments.

Which Solution to Choose?

The choice depends on several key factors:

• Number of disks: RAID 6 or equivalent is recommended once you exceed four disks.
• Data criticality: For highly sensitive data, opt for RAID 10 or ZFS.
• Performance needs: RAID 10 offers superior performance, while RAID 6 provides a good balance of security and capacity.

RAID 5: A Viable Option for Non-Critical Data

In scenarios where capacity is a priority and data loss would have minor consequences, a RAID 5 with 8 to 16 disks can be considered. However, the risk of multiple failures is real. This option should be chosen with full awareness and regular backups.

My Personal Opinion: Prioritize Security Above All

In the life of an IT consultant or system administrator, two things are invaluable: time and data. Opting for RAID configurations based solely on cost savings can be disastrous in case of failure. In my opinion, once you exceed four disks, always prioritize two-disk fault tolerance with technologies like RAID 6 or equivalent. This allows you to focus on other priorities without worrying about data loss. In IT, true peace of mind lies in robust security.

There’s also Murphy’s Law to consider: Anything that can go wrong will go wrong. If you already have a failed disk under reconstruction, the probability of a second disk failing is no longer a distant risk—it’s simply a matter of when, not if. With the added stress placed on the remaining disks during rebuild, the likelihood of another failure increases exponentially. Relying on single-disk fault tolerance in such scenarios is gambling with your data, and Murphy’s Law ensures that the odds are not in your favor.

Conclusion: Prioritize Security and Peace of Mind

Choosing the right RAID solution is a crucial balance between performance, capacity, and security. While RAID 5 remains a viable option for non-critical data, it becomes risky for large configurations. Alternatives like RAID 6, ZFS, or SHR-2 provide superior protection. Investing in a robust solution from the start not only preserves your data but also grants an invaluable asset: peace of mind.