Is RAID 5 or RAID 6 Best For You and Your NAS?

When setting up a NAS, one of the most important and long-lasting decisions you’ll make is choosing the right RAID level. This choice directly impacts how much protection you have against drive failures, how much usable storage space you retain, and how long rebuilds will take when things go wrong. Among the most debated options are RAID 5 and RAID 6, both of which use parity for data protection but differ in how much risk they can tolerate. RAID 5 offers single-drive failure protection with better capacity efficiency, while RAID 6 provides dual-drive fault tolerance at the cost of more storage overhead and longer rebuild times. It’s worth noting that although you can graduate a RAID 5 into a RAID 6 later if your needs change, this is a slow and resource-heavy process. On the other hand, RAID 6 cannot be reversed back into RAID 5, so it’s a decision that requires careful planning from the outset. The balance of speed, safety, capacity, and risk tolerance will determine which configuration is truly best for your setup.

IMPORTANT – It is essential to understand that RAID, whether RAID 5 or RAID 6, should never be considered a true backup solution. RAID protects against drive failures, but it cannot safeguard you from accidental deletion, malware, hardware faults beyond the disks, or disasters like fire and theft.

The TL;DR Short Answer – Over-Simplified, but….

• Under 8 Bays = RAID 5
• 8 Bays or Over = RAID 5, or RAID 6 with Bigger HDDs
• 12 Bays or Over = RAID 6

If you are looking for simplicity, RAID 5 will usually give you the best balance of speed, storage efficiency, and cost, but it comes with higher risk. RAID 6 is slower to rebuild, consumes more usable capacity, and involves heavier parity calculations, but it provides a much stronger safety net against drive failures. For smaller arrays with modest drive sizes, RAID 5 can be entirely sufficient, especially when paired with reliable backups. However, as drive capacities continue to grow and rebuild times stretch into days, RAID 6 becomes more attractive because it can withstand the failure of two drives without losing the array. In essence, RAID 5 is about maximizing space and performance with a moderate level of safety, while RAID 6 is about maximizing resilience and peace of mind at the expense of capacity and speed. Choosing between them comes down to how valuable your data is, how large your drives are, and how much risk you are willing to tolerate during rebuild windows.

For systems with fewer than 8 bays, RAID 5 will usually be sufficient unless you are running especially large-capacity drives or operating at a business scale where data loss cannot be tolerated. Once you reach 8 bays or higher, RAID 6 should be seriously considered, as the chances of a second drive failing during a rebuild increase along with the overall storage pool size and the scale of potential loss. At 12 bays and beyond, RAID 6 is effectively mandatory, as relying on RAID 5 at that scale means gambling with too many points of failure and too much at stake if something goes wrong.

RAID 5 vs RAID 6 – Build Time and RAID Recovery Time

The initial creation of a RAID array, sometimes called synchronization or initialization, is one of the first differences you’ll notice between RAID 5 and RAID 6. A RAID 5 setup generally completes its initial build faster because it only has to calculate and assign a single parity block across the drives. RAID 6, by contrast, has to generate and distribute two independent parity values on every stripe, which increases the workload on the system. This means that on a fresh setup, RAID 6 will take longer to complete the synchronization process before the array is fully operational, though this is usually a one-time inconvenience at the beginning of deployment. For home and small office setups, this extra build time might not matter too much, but in larger systems with many terabytes of data, it can mean several hours or even days of extra initialization work compared with RAID 5.

The difference becomes more significant when a drive fails and a rebuild is needed. In RAID 5, the system only needs to reconstruct the missing data using the surviving disks and a single parity calculation, which usually makes recovery noticeably faster. RAID 6, however, must perform double parity calculations and restore both sets of parity information onto the replacement drive, extending the recovery window. On large modern HDDs where rebuilds can take dozens of hours, or sometimes multiple days, this extra time becomes a major factor. The trade-off is that RAID 6 offers much stronger resilience while this rebuild is in progress, because the system can continue to operate and survive even if another disk fails during the process. In other words, RAID 5 rebuilds faster but carries more risk, while RAID 6 rebuilds slower but provides a crucial safety margin during the vulnerable degraded state.

Here is a recent video (using the UniFi server platform) that talks about RAID 5/6 vs RAID 10 build times and parity from 777 or 404:

RAID 5 vs RAID 6 – Protection and Vulnerability

The most important factor when comparing RAID 5 and RAID 6 is how well they protect data when drives fail. RAID 5 uses single parity, meaning the system can survive one drive failure without losing data. However, if a second drive fails during the rebuild, the entire array is lost. RAID 6 adds dual parity, which allows the system to tolerate the loss of two drives simultaneously. This extra layer of protection is especially valuable during rebuild windows, which can take many hours or days on modern high-capacity HDDs. In practice, RAID 6 dramatically reduces the risk of catastrophic data loss, at the expense of slower rebuilds and less usable capacity. A subtle but often overlooked vulnerability is the issue of batch manufacturing. Many users buy multiple drives at once, often from the same supplier, meaning the disks may come from the same production batch. If there was a hidden flaw introduced during manufacturing, it is possible that more than one disk could develop problems around the same time. With RAID 5, this creates a dangerous scenario: a second disk failure during a rebuild results in complete data loss. RAID 6 provides a safety margin against these correlated failures by protecting the array even if two drives fail close together in time. Another major risk comes from unrecoverable read errors (UREs) that can occur during rebuilds. Because every sector of every remaining drive must be read to restore the lost disk, the chance of encountering a read error rises significantly with larger drives. In RAID 5, a single URE during rebuild can corrupt the recovery process, whereas RAID 6 has an additional layer of parity to compensate, making it much more reliable during rebuilds. This is especially important in arrays of 8 or more drives, where the probability of encountering at least one problematic sector grows. For users with large arrays or very high-capacity drives, RAID 6’s extra fault tolerance is the difference between a successful rebuild and complete data loss.

RAID 5 vs RAID 6 – Capacity and Price per TB

One of the clearest differences between RAID 5 and RAID 6 lies in how much usable capacity you end up with. RAID 5 only sacrifices the equivalent of a single drive’s worth of storage to parity, which makes it the more space-efficient option. In a six-bay system with 10TB drives, RAID 5 would deliver 50TB of usable storage, while RAID 6 would only provide 40TB. That 10TB difference can be substantial when you are working with large libraries of data such as media collections, surveillance archives, or backups. For users trying to maximize every terabyte of their investment, RAID 5 makes the most efficient use of available space. However, RAID 6’s higher storage overhead translates directly into a higher effective cost per terabyte. Since two drives are always reserved for parity, the total usable space is reduced, and the price you pay for storage per TB goes up. For small home users, this may feel like wasted potential, but the trade-off is the additional layer of fault tolerance. In environments where the cost of downtime or data loss far outweighs the cost of an extra disk, RAID 6 provides stronger long-term value despite the higher price per terabyte. Ultimately, the decision comes down to whether you are more concerned with minimizing cost and maximizing space, or ensuring redundancy and peace of mind.

RAID 6 vs RAID 5 + Hot Spare Drive?

Some users prefer to run RAID 5 with a dedicated hot spare drive rather than choosing RAID 6 outright. In this setup, a single extra disk sits idle until one of the active drives fails, at which point the spare is automatically used for the rebuild. This reduces the amount of time the array spends in a degraded and vulnerable state, since the rebuild begins immediately without waiting for a replacement disk to be manually installed. While this approach still leaves you with only single-drive fault tolerance, it can feel like a middle ground between RAID 5 and RAID 6. In terms of capacity, RAID 5 with a hot spare sacrifices the same amount of usable space as RAID 6, but it does not provide the same dual-drive protection. For arrays of six to eight drives, this compromise can make sense if you prioritize capacity efficiency and faster automated recovery, but once you move into larger-scale storage systems, RAID 6 remains the safer and more resilient option.

RAID 5 vs RAID 6 – Conclusion and Verdict

When choosing between RAID 5 and RAID 6, the decision comes down to weighing efficiency against resilience. RAID 5 is faster to rebuild, provides more usable storage, and costs less per terabyte, which makes it well suited to smaller NAS setups or users who prioritize capacity and speed. RAID 6, on the other hand, offers stronger protection against drive failures, making it far more reliable for larger arrays and higher-capacity drives where rebuild times are long and risks multiply. The general consensus is that RAID 5 can still be a smart choice for arrays under eight bays, but RAID 6 becomes the clear recommendation for systems of eight drives or more, and an essential requirement at twelve drives and beyond. Above all else, it is critical to remember that RAID is not a backup. Neither RAID 5 nor RAID 6 will protect you against accidental deletion, ransomware, hardware faults beyond the disks, or disasters such as fire or theft. RAID is a safety net that improves availability, but it must always be paired with a proper backup strategy if your data truly matters.

 

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