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ServeTheHome’s RAID Reliability Calculator is useful for comparing RAID layouts under a shared set of drive, read-error, array-size, and rebuild assumptions. It is not a forecast of how long your array will last, and it cannot account for every way a storage system can fail. Treat its results as directional planning estimates—not guarantees—and keep independent, tested backups.
What the calculator measures—and what it does not
The tool models selected drive-failure and read-error risks for RAID 0, RAID 1, RAID 10, RAID 5, RAID 6, RAID-Z3, and RAID 50. ServeTheHome describes it as a simple Poisson-based model intended to be directionally useful, not a complete or highly accurate reliability analysis. It excludes failures in components such as controllers, motherboards, and power supplies, and labels the calculator beta. See the calculator’s explanation and limitations.
Mean Time To Data Loss (MTTDL) is a statistical estimate of the average time until a modeled system reaches a condition that exceeds its redundancy or otherwise loses data. It does not mean a particular array will survive for that many years. An output of 100,000 years is not a lifespan prediction; for an individual deployment, it is best understood as a model-dependent comparison figure.
- MTBF is mean time between failures for a component or system, usually expressed in hours. Manufacturer MTBF is a statistical specification, not a promise that an individual drive will last that long.
- AFR, or annualized failure rate, expresses a modeled or observed failure rate on an annual basis. It is not interchangeable with a drive’s warranty period.
- MTTDF is the calculator’s Mean Time To Data Failure measure, shown separately from the read-error contribution.
- Bit Error Rate MTTDL represents a modeled contribution from read-error risk.
- Availability concerns whether a system is online and serving data. A degraded array or failed controller can interrupt service without necessarily destroying data.
- Durability is whether data remains recoverable, including through separate backups. RAID alone does not provide that protection.
The calculator’s MTTDL combines modeled risks; it should not be read as a complete probability distribution or as a guarantee about a particular storage system.
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How to use the ServeTheHome calculator
- Open the ServeTheHome calculator article and use its embedded calculator, or open the calculator directly.
- Select or enter the drive’s Mean Time Between Failures (MTBF). Use an appropriate figure for the drive and workload; do not assume a manufacturer value precisely describes your fleet.
- Select the Nonrecoverable Error Rate (also called an unrecoverable read-error rate in some specifications). Match the unit and specification shown by the calculator.
- Enter Drive Capacity and select its unit. Be consistent about whether a number is raw or usable capacity.
- Enter Sector Size and its unit. The calculator’s default example displays 4,096 bytes; use the value appropriate to the drives and interpretation you are modeling.
- Enter the Quantity of Disks in the modeled arrangement.
- Enter Volumes. The field is exposed by the tool, but its text explanation does not fully define the term. Treat it as a calculator-specific configuration parameter affecting modeled volume size and exposure. Do not substitute the number of shares, filesystems, datasets, drive letters, or physical arrays unless you have confirmed that this matches the tool’s intended meaning.
- Enter Volume Rebuild Speed (MB/s), preferably from a representative rebuild on the actual system. Use a sustained, conservative rate, not a brief peak.
- Review the calculated rebuild time and the RAID comparison table. Compare layouts only while holding the assumptions constant, and record those assumptions beside the results.
The live calculator displays fields including “Select Mean Time Between Failures,” “Nonrecoverable Error Rate,” “Drive Capacity,” “Sector Size,” “Quantity of Disks,” “Volumes,” and “Volume Rebuild Speed (MB/s).” Its defaults are example values, not recommendations. The visible example uses an MTBF of 1,200,000 hours and 4,096-byte sectors, and reports a rebuild time of about 71.46 hours; those outputs are specific to that displayed scenario, not universal performance or reliability guidance.
Choosing assumptions that are useful
MTBF and AFR
Start with the drive manufacturer’s published specification, but treat it as an input to test rather than ground truth. Manufacturer figures may use different conditions and assumptions, so do not compare unlike metrics as though they were direct predictions of field failure. ServeTheHome notes that users can enter their own MTBF figures and that real-world values may need to be adjusted downward. A year is approximately 8,760 hours for the calculator’s purposes.
For a first-pass comparison, run more than one scenario: a favorable case, a reasonable planning case, and a conservative case. If plausible changes to MTBF reverse the ranking or change the result dramatically, that sensitivity is itself important information.
Nonrecoverable read errors and sector size
The read-error input helps the model estimate the chance of encountering a read problem while data is being read and reconstructed. It is not a direct prediction that a drive will fail during a rebuild, nor does a specification guarantee how a particular drive behaves. Treat it as a model parameter and vary it in sensitivity checks.
Sector size matters because the modeled amount of data and the read-error exposure depend on how the calculator represents the storage. Newer disks commonly use 4 KB sectors, but drive presentation and formatting can complicate what a user sees. Confirm the relevant drive specification and calculator units rather than assuming every device reports sectors in the same way.
Rebuild speed and rebuild time
A failed drive can leave an array degraded until reconstruction finishes. During that period, redundancy may be reduced, so rebuild duration affects modeled exposure to another drive failure or read error. A faster rebuild generally shortens that window, but the calculator’s single speed value cannot capture every operational detail.
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Use a measured sustained rebuild rate from your own platform when possible. If you cannot measure it, choose a conservative estimate and model a slower case as well. A rebuild running while the array serves production I/O may be throttled or delayed; controller throughput, access patterns, competing workload, bad sectors, and rebuild priority all matter. HDD and SSD arrays should not automatically be assigned the same rate. ServeTheHome gives 65–140 MB/s as an example range for modern 3.5-inch SATA disks, not a universal current specification.
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Drive count, capacity, and volume count
Keep the physical drive count and capacity consistent with the layout you are comparing. A larger modeled array can change both usable capacity and failure exposure. The calculator reports formatted capacity, but capacity conventions and RAID overhead can make that figure differ from raw manufacturer capacity or the usable space shown by an operating system.
The “Volumes” field deserves particular caution: because the page does not fully define it in its text, do not infer that it maps directly to a filesystem, NAS share, or independent array. If changing it substantially changes the results and you cannot determine the intended mapping in the interface or its documentation, treat the comparison as uncertain rather than choosing the value that produces the preferred answer.
How to read the output table
- Formatted Capacity (GB): Estimated usable capacity for the modeled layout.
- Mean Time To Data Failure (MTTDF): The calculator’s modeled failure measure before the additional bit-error component.
- Bit Error Rate MTTDL: The modeled read-error contribution.
- Mean Time To Data Loss (MTTDL): The combined modeled result, shown in hours.
- MTTDL (Years): The hour-based result converted to years.
- Rebuild time and drive-related values: The calculator also displays intermediate figures such as AFR, MTBF, volume size, sector size, sectors per disk, formatted disk size, bits per disk, bit-error rate, clean-read probability, and expected hard or silent error rate.
Do not select a layout solely because it has the largest MTTDL number. That ranking reflects only the assumptions and mechanisms represented by the model. Capacity efficiency, performance, rebuild behavior, failure tolerance, platform support, expansion options, operational complexity, and recovery requirements all affect the real design decision.
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Extremely large results—millions or billions of years, for example—are especially easy to misread. They do not establish that an individual array is effectively immortal. They usually mean that, under the simplified assumptions, the modeled event is rare relative to the unit of time displayed. Use such values chiefly for relative comparison between scenarios, and do not let them weaken backup or monitoring plans.
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What the listed RAID levels mean in practice
The descriptions below summarize common layout behavior; they are not additional conclusions produced by the calculator. Exact capacity, performance, and failure behavior depend on implementation and configuration.
| Layout | Redundancy | Capacity tendency | Performance tendency | Main concern |
|---|---|---|---|---|
| RAID 0 | None | High usable capacity | Can improve parallel throughput | Any drive failure can destroy the array. |
| RAID 1 | Mirroring | Low efficiency relative to raw capacity | Simple writes; reads may be served from mirror members | Failure of the remaining mirror member before recovery. |
| RAID 10 | Striped mirrors | Moderate | Often strong general-purpose performance | Failure tolerance depends on which drives fail; multiple failures in one mirror pair are critical. |
| RAID 5 | Single parity | Good capacity efficiency | Parity writes add overhead | A second failure or unrecoverable read problem during reconstruction can be consequential. |
| RAID 6 | Dual parity | Lower efficiency than single parity | More parity overhead | Rebuilds can be long, and protection still depends on implementation and operations. |
| RAID-Z3 | Triple parity in a ZFS layout | Layout-dependent | Depends on ZFS configuration and workload | It is not simply interchangeable with generic RAID 6; filesystem and rebuild behavior differ. |
| RAID 50 | Striped RAID 5 groups | Moderate, dependent on group design | Striping can improve parallelism | A second drive failure in one RAID 5 group can make that group fail. |
For RAID 10, “two-drive tolerance” is not a general guarantee: two failed drives may be survivable if they are in different mirror pairs, but not if both belong to the same pair. Likewise, RAID 5, RAID 6, RAID-Z3, and RAID 50 involve different parity and implementation behaviors. A calculator’s simplified labels do not replace the filesystem or controller’s documentation.
A practical comparison workflow
- Define the real candidates. Use the same number and class of drives where possible, and specify the actual RAID or filesystem implementation—not just a label.
- Write down assumptions. Record MTBF, error-rate units, drive capacity, sector size, disk count, volumes, and rebuild rate for each run.
- Use a conservative rebuild estimate. Prefer measured performance under a representative workload; otherwise test slower rebuild cases.
- Vary the uncertain inputs. Compare a favorable, expected, and pessimistic scenario. Pay special attention to rebuild time, error-rate assumptions, and the ambiguous volume parameter.
- Compare the whole design. Look at modeled MTTDL alongside usable capacity, likely performance, expansion, spare availability, restore objectives, and the consequences of downtime or data loss.
- Keep a record. Save the configuration and date with the output. A result without its input assumptions is not a useful design artifact.
For example, if comparing a six-drive single-parity layout with a dual-parity alternative, do not change the drive assumptions or rebuild rate between runs. First compare them under the same plausible scenario, then repeat both with a slower rebuild and more conservative drive assumptions. The point is not to manufacture one definitive year-count; it is to see whether the trade-off remains acceptable across reasonable cases.
Why the model may be optimistic or incomplete
Drives do not always fail independently
A simple model can understate risk when disks share a manufacturing batch, deployment date, operating hours, firmware, temperature, vibration, power source, enclosure, or backplane. A power event, overheating incident, firmware defect, or common environmental problem can affect several drives at once. Mixing models or batches may reduce some shared risks, but it does not guarantee independence.
Failure rates are not constant in every real fleet
MTBF-based calculations simplify a complicated failure process. Real devices can show early failures, age-related wear, workload effects, latent defects, firmware incidents, and failures that become visible only when a rebuild reads data that has not recently been accessed. A single MTBF value cannot describe all of these patterns.
Rebuilds are not always smooth or uninterrupted
The model represents rebuild using a rate, but actual reconstruction can be throttled, interrupted, delayed, or slowed by workload and errors. The time until a rebuild begins also matters. A hot spare, prompt alerting, and a practiced replacement procedure can reduce operational delay; none makes the array immune to another failure.
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Important system and human failures are outside the drive model
Controller, motherboard, power supply, enclosure, cooling, cabling, software, and administrator failures can interrupt or destroy access to an array. The calculator’s own explanation excludes several such components. Accidental deletion, ransomware, corruption replicated across systems, theft, fire, flood, and incorrectly executed recovery operations are also not solved by choosing a higher RAID level.
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A stated nonrecoverable read-error rate is a specification used by the model, not a promise that every drive will behave exactly to that figure or that it predicts data loss for every rebuild. Checksums and scrubbing can help detect latent corruption, but their behavior depends on the storage stack; scrubs also read substantial data and should be part of an operational plan.
RAID is not a backup
RAID can maintain access through some drive failures, depending on its layout. It does not create an independent historical copy. Keep versioned backups with at least one copy outside the array’s failure domain, such as an off-site or otherwise isolated copy. Protect backup credentials, define retention, monitor backup jobs, and periodically test restoration—not just backup completion.
For valuable data, pair the array with monitoring and alerts, compatible spare-drive planning, documented replacement and recovery steps, and restore tests. A NAS, snapshot feature, or replication target can be useful, but a synchronized deletion or corruption may propagate; these features should not be mistaken for independent, versioned backups.
When this calculator is—and is not—enough
Use it for preliminary comparisons, teaching the effect of drive count or rebuild time, and testing how different assumptions affect modeled drive-related risk. It is not enough by itself for business-critical design, unusual erasure coding, mixed or correlated drive populations, heavily loaded rebuilds, or systems where controller, enclosure, power, filesystem, and recovery behavior dominate the risk.
For consequential infrastructure decisions, combine vendor reliability documentation with platform- and filesystem-specific guidance, fleet or operational data where available, and explicit availability and recovery objectives. More rigorous reliability work may require a Markov or Monte Carlo model that represents failure dependencies, degraded states, repairs, and component behavior. Do not infer an exact mathematical implementation beyond what ServeTheHome documents.
ServeTheHome’s model is identified as beta. For tool issues or suggestions, its article points readers to the official forum thread. The calculator is best treated as one planning aid among several, not as an authority on the reliability of a complete storage system.
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