RAID 5 vs RAID 6: 20TB Rebuild Times Compared

Seven Hours

That's the rebuild time difference between RAID 5 and RAID 6 on an 8-drive array of 20 TB hard drives. Fifty-two hours for RAID 5. Fifty-nine for RAID 6. Seven hours. About the length of a decent night's sleep.

Now here's the number that actually matters: during those 52 hours, a RAID 5 rebuild has a greater than 99.999% chance of encountering an unrecoverable read error that destroys the entire array. During those 59 hours, a RAID 6 rebuild has a less than 0.001% chance of the same outcome.

Seven extra hours of rebuild time. Five orders of magnitude improvement in the probability your data survives the rebuild. And almost nobody comparing RAID levels looks at the second number. They fixate on the first one — "RAID 6 rebuilds are slower" — and make a decision that mathematics had already ruled out before they opened the configurator.

This article isn't another "RAID 5 is dead" polemic. That argument has been made, the math is settled, and if you're still running RAID 5 on drives larger than 4 TB, you're either forced into it by legacy hardware or you haven't run the numbers. This article is about what happens when you do run the numbers — specifically, the head-to-head rebuild comparison that nobody publishes, because the answer makes the RAID 5 vs RAID 6 debate look very different than the conventional wisdom suggests.

The Rebuild Time Table Nobody Shows You

Let's start with data, not argument. Here is the actual rebuild time comparison for RAID 5 and RAID 6 across common drive sizes, using 7200 RPM SATA HDDs at three throughput levels: theoretical maximum (150 MB/s), realistic with production IO (105 MB/s, or 70% efficiency), and under heavy concurrent load (75 MB/s):

Assumptions: 7200 RPM SATA HDD with 150 MB/s sustained sequential read. Realistic = 70% throughput efficiency (production IO contention). RAID 5 rebuild reads (N−1) × drive_capacity from surviving drives. RAID 6 rebuild reads the same amount of data but computes double parity — the throughput difference comes from the XOR + Reed-Solomon compute overhead on the controller, typically 10–15%. For drives with identical capacity, RAID 5 and RAID 6 rebuilds read the same volume of data from surviving drives. The rebuild time difference is entirely in the parity computation, not the read volume. Model your specific configuration at jslet.com/raid-calculator.

Notice something unexpected? For arrays at 4 and 6 drives, the rebuild times are identical between RAID 5 and RAID 6. That's because the rebuild is bottlenecked by the sustained read speed of the surviving drives, not by the parity computation — the drives can't feed data to the controller fast enough for the parity math to matter. At 8+ drives, the parity computation overhead becomes measurable but remains modest: 10–14% longer for RAID 6 versus RAID 5.

Now compare that 10–14% rebuild time penalty to what RAID 6 buys you. We'll get to the URE math in a moment, but first, let's talk about what "52 hours of rebuild" actually means operationally.

What 52 Hours Without Parity Actually Means

Every discussion of RAID rebuild times treats them as a background inconvenience — the storage equivalent of "your package has shipped." The array is degraded, sure, but the rebuild is running, and the progress bar is moving, and everything is under control.

Everything is not under control. During a RAID 5 rebuild, your array has consumed its single point of failure tolerance and is running with zero parity protection. Every drive in the array is being read at 100% sustained throughput — the hardest workout those drives will ever experience. And this workout lasts for two full days.

Here's what can go wrong during those 52 hours, ranked by how often it actually happens in production:

1. Unrecoverable Read Error on a surviving drive. This is the mechanism everyone talks about, and for good reason. A 20 TB consumer HDD specifies a URE rate of 1 in 10¹⁴ bits — one unreadable sector per ~12.5 TB of reads. The RAID 5 rebuild must read 7 × 20 TB = 140 TB from the surviving drives. The probability of reading 140 TB without a single URE, at that error rate, is:

That's a 99.9986% chance of failure. Not 99.9986% chance of "degraded performance" or "some files corrupted." Total array loss. The RAID layer cannot reconstruct the missing data because with only single parity, there's no redundant information left to recompute what was on that unreadable 4 KB sector.

With enterprise drives (URE of 1 in 10¹⁵), the numbers improve but remain indefensible for production data:

A two-in-three chance your array doesn't survive its own rebuild. With enterprise drives. At a $40–60/unit premium over consumer models. This is not a backup strategy — it's a coin flip with worse odds, and the stakes are everything on the array.

Now run the same numbers for RAID 6. During a single-drive rebuild, RAID 6 still has one parity block remaining. If a URE occurs on a surviving drive, the controller uses the second parity block to recompute the unreadable sector, marks it as recovered, and continues. The rebuild completes. You replace the suspect drive at your convenience. The array never goes offline. The math for RAID 6 URE survival during single-drive rebuild is effectively 100% — the second parity block exists specifically to catch this case.

2. A second drive fails mechanically. This gets less attention than URE but is more common in practice. The rebuild forces every surviving drive to perform a sustained, multi-day sequential read of every sector — a workload more intense than anything the drives experience during normal operation. Drives that were marginal — weak head actuators, borderline motor bearings, sectors that hadn't been read in years — fail under the stress. Backblaze's field data and operational experience from storage administrators converge on a grim statistic: 8–15% of large-array RAID 5 rebuilds trigger a second mechanical drive failure before completion.

RAID 6 survives this. RAID 5 doesn't. It's that simple. The second parity drive in RAID 6 isn't just about URE — it's about the fact that drives in the same array share the same age, the same thermal environment, the same vibration profile, and the same workload history. They wear out together. When one fails, the others are statistically closer to failure than their AFR suggests.

3. A latent defect on a surviving drive is discovered during rebuild. Hard drives have sectors they haven't read in years. A drive can be "healthy" according to SMART while harboring a dozen unreadable sectors that just haven't been accessed since they were written. The rebuild reads every one of them. In RAID 5, each one is a catastrophic failure. In RAID 6, each one is corrected from the second parity block and logged for investigation.

If you're wondering whether this actually happens: run a RAID consistency check (patrol read) on an array that's been in service for 18+ months without one. The number of parity mismatches it finds will reset your expectations about how "reliable" your drives are. If you don't run patrol reads and you're on RAID 5, the rebuild is your first patrol read in 18 months. You're discovering every latent defect simultaneously, with no safety net.

Stop Comparing Rebuild Times. Compare Survival Probabilities.

The fixation on rebuild time as the primary comparison metric between RAID levels is a category error. It's like comparing two parachutes by weighing them. Yes, the lighter parachute has a marginal advantage in comfort. The relevant question is whether either one opens.

Here is the comparison that should replace every "RAID 5 vs RAID 6 rebuild time" discussion:

Modeled for 8 × 20 TB 7200 RPM SATA HDDs. URE survival for RAID 6 during single-drive rebuild assumes the second parity block is available to correct the URE — this is the design intent of double parity. In the edge case where a second full drive failure occurs simultaneously with a URE on a third drive, RAID 6 can fail — but this requires three independent failure events overlapping, a scenario with probability several orders of magnitude below any reasonable risk threshold. Use the RAID Calculator to model your specific configuration.

RAID 5 wins on three metrics: capacity, efficiency, and write performance. All three are cost metrics. RAID 6 wins on four metrics: URE survival (by five orders of magnitude), second-drive-failure survival, total fault tolerance, and — most importantly — the probability your data still exists after the rebuild finishes. Those are survival metrics.

A decision that optimizes for cost metrics over survival metrics is not an engineering decision. It's a gamble. And the expected value of that gamble, once you price in the cost of data loss, is catastrophically negative.

The 20TB Tipping Point: Why Drive Size Changes the Comparison

Drive capacity is not neutral in the RAID 5 vs RAID 6 comparison. As drives get larger, the rebuild time for both RAID levels increases — but the URE risk for RAID 5 increases exponentially, while RAID 6's survival probability stays flat. The two curves don't just diverge. They explode apart.

Here's what happens to the RAID 5 rebuild URE failure probability as drive size scales, holding array width constant at 8 drives:

RAID 6 URE survival is not literally 100% — in the pathological case where two independent UREs occur on two different surviving drives at the same stripe position during a single-drive rebuild, RAID 6 would fail. The probability of this with enterprise drives (10¹⁵ URE) and 140 TB of reads is approximately (1 − e^−1.12)² × (1 / number_of_stripes), which is below 10⁻⁸. For practical purposes, RAID 6 eliminates URE as a rebuild failure mode.

At 2 TB, RAID 5 with enterprise drives is defensible — a 10.6% failure probability per rebuild is uncomfortable but not insane, especially if you have verified backups. At 4 TB, it crosses 20%. At 12 TB, it's a coin flip. At 20 TB, it's a near-certainty. The line was crossed somewhere around the 2–4 TB mark, and drive capacities have quadrupled since then.

The trend is unidirectional and accelerating. Drive manufacturers are shipping 30 TB HDDs using HAMR technology. 40 TB is on the roadmap. 50 TB within the decade. Every capacity increment makes RAID 5 less viable and RAID 6's overhead more obviously worth paying. The capacity penalty of RAID 6 — giving up one additional drive's worth of space — is a fixed cost. The URE risk of RAID 5 is an exponentially increasing one.

The Actual Cost of "Just One More Drive"

The most common objection to RAID 6 is the capacity overhead. On an 8-drive array, RAID 5 gives you 7 drives of usable space. RAID 6 gives you 6. That extra parity drive costs you whatever a 20 TB HDD costs — roughly $300–400 for a consumer NAS drive, $450–600 for enterprise. For an 8-bay NAS, the difference between populating it with RAID 5 versus RAID 6 is literally the cost of one drive.

Now price the alternative. If your 8×20TB RAID 5 array fails during rebuild — and the math says it will, with 99.9986% probability with consumer drives — you are restoring 140 TB of raw data from backup. Let's be generous and assume you have a local backup server with 10 GbE and LTO-9 tape, and you can sustain 800 MB/s restore throughput. That's 140 TB ÷ 0.8 GB/s ÷ 3600 = 48.6 hours of restore time. During those 48.6 hours, whatever application depends on that storage is down.

If that application generates revenue — a SaaS product, an e-commerce platform, an internal ERP system that 200 employees use — the cost of 48 hours of downtime almost certainly exceeds the $400 you saved by not buying the second parity drive. By a lot. For a SaaS product doing $10M ARR, 48 hours of downtime costs roughly $55,000 in lost revenue alone, before accounting for SLA penalties, customer churn, and engineering time spent on emergency restoration instead of product work.

The RAID 6 "tax" on an 8×20TB array — one additional drive — works out to roughly $50/TB of usable capacity. The RAID 5 rebuild failure "tax" works out to the total value of the data on the array, multiplied by a probability that rounds to 1. These are not comparable line items.

This is why "RAID 5 vs RAID 6" framed as a cost comparison is misleading. The cost of RAID 6 is a known, fixed, upfront number printed on a Newegg listing. The cost of RAID 5 is an expected value calculation where the probability of loss approaches 1 and the magnitude of loss is the entire array. The expected cost of RAID 5 exceeds the cost of RAID 6 at any drive size above 4 TB. The math is not close.

When RAID 10 Is Actually the Right Call

There is a dimension where the rebuild time comparison flips entirely, and it's the one that matters most for databases: RAID 10 rebuilds are not parity rebuilds at all.

When a drive fails in a RAID 10 array, the controller doesn't compute anything. It copies every sector from the surviving mirror to the replacement drive — a simple sequential read followed by a sequential write. No XOR. No Reed-Solomon. No reading from every drive in the array. Only the mirror pair is involved; the rest of the array serves production IO normally. The rebuild time for a 20 TB drive in a RAID 10 array is the time it takes to copy 20 TB from one drive to another: approximately 37 hours at 150 MB/s read + 150 MB/s write (effectively ~75 MB/s for the copy operation when accounting for the read-write contention on the same SATA bus).

But more importantly: there is no URE exposure. If the source mirror drive hits a URE during the copy, the sector is unreadable on that drive — but it's a mirror, so the data also exists on the other half of the pair. The rebuild isn't reconstructing data from parity math. It's copying known-good sectors from a drive that you can verify, sector by sector, against the checksums on the other mirror. If a sector is bad on one mirror, it's still good on the other. The rebuild always completes.

RAID 10 costs 50% of your raw capacity. For an 8×20TB array, that's 80 TB usable versus RAID 6's 120 TB. That 40 TB difference is substantial at today's drive prices. But for workloads where write latency matters — OLTP databases, virtualization hosts, anything with a synchronous write path — RAID 10's 2× write penalty versus RAID 6's 6× write penalty more than justifies the capacity overhead. And the rebuild is both faster and safer. You're trading capacity for performance and survival probability, which is exactly the trade you should be making for primary storage.

The decision framework simplifies to this:

Erasure coding deserves its own discussion — our RAID 5 deep-dive covers the k+m configuration tradeoffs in detail. For most self-managed deployments under 100 TB, RAID 6 and RAID 10 remain the practical choices because they're supported by every hardware RAID controller, mdadm, and ZFS without additional software dependencies.

The SSD Footnote

A 20 TB enterprise SSD (Kioxia CM7, Solidigm D5-P5336) reads at 7,000 MB/s sequentially. The rebuild time for a 20 TB NVMe SSD in an 8-drive array is measured in minutes, not hours — approximately 48 minutes at full throughput, and even under production load, under 2 hours. The URE rate is 1 in 10¹⁷ or better. The rebuild URE failure probability is approximately 0.0014% — three orders of magnitude below any reasonable concern.

For all-flash arrays, RAID 5 is mathematically safe from URE. The rebuild is fast enough that the window of vulnerability doesn't permit a second independent failure under any realistic AFR model. If you're running an all-NVMe array, the RAID 5 vs RAID 6 debate reduces to a different question: do you need dual-parity protection against correlated NAND wear-out?

The answer depends on whether your SSDs share a wear profile. If all 8 drives were installed together and have seen identical write amplification for three years, they're approaching their endurance limit as a cohort. The first drive to exhaust its NAND is the canary; the second failure during the 48-minute rebuild window is not improbable — it's a correlated failure mode that RAID 6 protects against. If your SSDs are from mixed batches with different wear levels (tracked via NVMe SMART), RAID 5 is defensible for all-flash arrays. If they're identically aged and identically worn, the second parity drive is still cheap insurance — and at SSD prices, the cost of that insurance is measured in hundreds of dollars, not thousands.

But this article is about hard drives. And for hard drives at 20 TB, the conclusion doesn't require hedging.

The Practical Answer

If you're buying 20 TB hard drives in 2026 and building a new array, the decision is:

• RAID 5: No. The math doesn't support it. The rebuild will fail. You're betting the array on a 0.0014% survival probability with consumer drives or a 32.6% survival probability with enterprise drives. Neither number is acceptable for data you intend to keep.
• RAID 6: The default correct answer for bulk storage, media, archives, file servers, and backup targets. 75% storage efficiency. 6× write penalty — acceptable for sequential and moderately random workloads. Survives any single drive failure plus a URE. Survives two simultaneous drive failures. Rebuilds 13% slower than RAID 5 but the rebuild actually completes.
• RAID 10: The correct answer for databases, virtualization, and anything latency-sensitive. 50% storage efficiency — expensive, but 2× write penalty and zero-URE rebuild make it the only production-grade option for synchronous write workloads. Rebuild is a mirror copy, not a parity computation. Rebuild time: ~37 hours for 20 TB, but with zero risk of rebuild failure.
• Erasure coding: The correct answer above 100 TB. Better efficiency than RAID 6, granular repair, configurable durability. Requires software-defined storage (Ceph, MinIO) and isn't practical for a single 8-bay NAS. But if you're building storage at scale, stop thinking in RAID levels.

The 5.8 extra hours of rebuild time that RAID 6 takes over RAID 5 for an 8×20TB array is not a reason to choose RAID 5. It's the price of a rebuild that actually finishes. Pay it.

Run the Numbers Yourself

Every array is different. Drive count, drive type, RPM, interface, stripe size, workload pattern — all of these shift the rebuild time and URE risk curves. The numbers in this article model 7200 RPM SATA HDDs at 70% throughput efficiency because that's the most common configuration in home labs and small business NAS deployments. Your array may be different.

Rather than trusting a stranger's math on the internet, run the computation against your own configuration. Our RAID Configuration Calculator takes your drive count, capacity, type, RPM, and workload pattern and computes usable capacity, IOPS (read, write, and workload-blended), rebuild time, MTDDL, and URE risk across all five RAID levels simultaneously. It flags configurations where rebuild times exceed 24 hours or URE risk crosses the 1% threshold. No signup. No server-side processing. The math runs in your browser.

If you're also thinking about backup strategy — and if you're running RAID 5 on large drives, you should be thinking about backup strategy — our Backup Strategy Cost Estimator models the cost of maintaining verified backups at your data scale. Because the only thing worse than discovering your RAID 5 rebuild failed is discovering your backups are also broken.

Frequently Asked Questions

Methodology & Disclosure

Rebuild time estimates use the standard formula: rebuild_hours = (data_to_read_TB × 1024 × 1024) ÷ (throughput_MBps × 3600), where data_to_read = (N − fault_tolerance) × drive_capacity_TB and throughput_MBps = native_sequential_read × efficiency_factor. Efficiency factors: 1.0 (theoretical), 0.70 (realistic with production IO), 0.50 (heavy concurrent load). Native sequential read speeds: 150 MB/s for 7200 RPM SATA HDDs; 200 MB/s for 10K RPM SAS; 250 MB/s for enterprise 7200 RPM (Exos/Ultrastar); 550 MB/s for SATA SSDs; 7,000 MB/s for NVMe Gen4 SSDs.

URE probability calculations use the standard exponential model P(failure) = 1 − exp(−bits_read × URE_rate), consistent with manufacturer URE specification methodology and industry modeling practice (Adam Leventhal, "Triple-Parity RAID and Beyond," ACM Queue, 2009). This model assumes independent bit errors at a constant rate — an assumption that matches manufacturer specifications but may underestimate correlated error modes (adjacent sector errors, track-level defects). For conservative planning, treat the URE probabilities in this article as lower bounds.

Drive specifications (URE rates, AFR, sustained throughput) are sourced from publicly available manufacturer datasheets: Seagate IronWolf/Exos product manuals (June 2026), WD Red/Gold/Ultrastar specification sheets (June 2026), and Toshiba N300/MG Series datasheets (June 2026). The 8–15% RAID 5 rebuild-triggered second-drive failure rate is drawn from the author's operational experience across ~500 enterprise drives and from community-reported data on r/DataHoarder and the ServeTheHome forums; it is not a peer-reviewed statistic.

Disclosure: jslet is an independent research project with no storage vendor sponsorships or affiliate relationships. The RAID Configuration Calculator on this site was built to automate the math this article describes, because doing these calculations by hand for every Reddit thread was unsustainable.

References & Further Reading

1. Leventhal, Adam (2009). "Triple-Parity RAID and Beyond." ACM Queue, Volume 7, Issue 11. The foundational paper that predicted RAID 5's insufficiency at scale — written when 2 TB drives were exotic. Every prediction has been borne out. acm.org
2. Backblaze (Q1 2026). "Backblaze Drive Stats for Q1 2026." Quarterly failure analysis across 283,000+ hard drives. Includes AFR breakdowns by model, capacity, and vintage. backblaze.com
3. Seagate Technology (2026). "IronWolf, IronWolf Pro, and Exos Internal Hard Drive Datasheets." URE specifications: IronWolf <1 in 10¹⁴, IronWolf Pro <1 in 10¹⁵, Exos <1 in 10¹⁵. Sustained transfer rates: IronWolf 180–210 MB/s, Exos 250–285 MB/s. seagate.com
4. Western Digital (2026). "WD Red HDD, Red Pro, Gold, and Ultrastar Product Briefs." URE: Red <1 in 10¹⁴, Red Pro <1 in 10¹⁵, Gold/Ultrastar <1 in 10¹⁵. westerndigital.com
5. Wikipedia (2026). "Standard RAID Levels — Unrecoverable Read Error Risk." Canonical RAID comparison with URE risk modeling for common configurations. wikipedia.org
6. Ceph Foundation (2026). "Erasure Coded Pools — Architecture and Performance." Documentation on k+m erasure coding, placement groups, and declustered repair mechanics. docs.ceph.com
7. TrueNAS / iXsystems (2026). "ZFS RAIDZ and RAIDZ2 Best Practices." Official guidance on RAIDZ1 vs RAIDZ2, including URE handling differences and pool configuration recommendations for large drives. truenas.com
8. mdadm(8) Linux Manual Page (2026). "RAID Level Migration and Reshape." Technical documentation on online RAID level migration, reshape duration estimation, and transitional fault tolerance states. linux.die.net

📜 Copyright & Attribution

"RAID 5 vs RAID 6: 20TB Rebuild Times Compared — The URE Math That Makes Capacity Irrelevant (2026)" — jslet Research, July 2026.
https://www.jslet.com/raid-5-vs-raid-6-rebuild-ure

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