RAID Reliability and MTTDL Calculator

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Environment preset:

Custom preserves the current values; another choice replaces all assumptions once.

RAID layout:

RAID 1 is modeled as a two-drive mirror; RAID 10 requires an even group width.

Drives per group:

Count active members in one protected set, excluding fleet-level staged spares.

drives

Repeated groups:

Use one for a single array or the total number of comparable protected groups.

groups

Drive capacity:

Use the smallest decimal capacity shared by the group members.

Drive AFR:

Enter 1.5 for a 1.5% annualized rate.

Rebuild duration:

This is the main degraded interval after replacement begins.

hours

Analysis horizon:

Use the period over which this exact layout and operating policy will be reviewed.

years

Repair-start delay:

Zero adds no delay beyond rebuild time.

hours

Staged spares:

Zero leaves the repair delay unchanged for every group.

spares

Spare activation:

This value is inert while staged spares are zero.

hours

Rebuild buffer:

Zero leaves the measured rebuild duration unchanged.

Correlation factor:

One is the neutral independent-drive baseline.

Include URE in risk score:

Off is neutral and leaves the headline score equal to annual fleet loss.

URE specification:

The custom exponent applies only when Custom exponent is selected.

Exponent 10^-

Patrol-read interval:

Thirty days is the neutral reference for the heuristic modifier.

days

Planned refresh cycle:

Zero records no planned refresh cycle and leaves the probability model unchanged.

years

Measure	Value	Planning read	Copy
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Scenario	Change	Annual fleet loss	Fleet MTTDL	Loss reduction	Copy
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Parameter	Value	Role	Copy
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Protected storage becomes most exposed after a drive has failed and before redundancy is restored. The array may remain available, yet another failure can overlap the degraded period and a full rebuild may have to read many terabytes from the surviving media. Detection delay, spare activation, rebuild duration, drive population, and group width all affect that exposure.

Mean time to data loss (MTTDL) converts a modeled loss hazard into a long-run comparison figure. It does not predict the service life of one array. A result of millions of years does not mean the array is guaranteed to survive for millions of years; it means the assumed constant-rate model produced a very small average hazard. Annual and multi-year loss probabilities are usually easier to use for planning.

RAID reliability terms and practical effects
Term	Meaning	Effect in the model
AFR	Annualized failure rate for one drive population.	Converted to a constant hourly drive hazard.
Degraded exposure	Repair-start delay plus buffered rebuild time.	Longer exposure raises overlapping-failure hazard.
Correlation factor	A multiplier for shared batch, firmware, enclosure, thermal, or operating conditions.	Scales the independent-drive overlap estimate from 1 to 4.
URE	Unrecoverable read error pressure while surviving media is scanned.	Shown separately and optionally blended into the risk score.
Repeated groups	The number of arrays or protection groups using the same design.	Turns per-group probability into fleet probability.

Layout width changes the number of overlapping failure combinations. Single-parity RAID 5 is exposed after one member fails. RAID 6 requires a three-drive overlap in this heuristic. RAID 1 is modeled as a two-drive mirror, while RAID 10 is treated as repeated mirror pairs inside each group. Those simplifications make layouts comparable, but they do not reproduce a specific controller's rebuild and fault-domain behavior.

Unrecoverable read errors and drive failures are different mechanisms. A URE estimate answers how likely the assumed bit error rate is to surface during a rebuild-sized read. The overlap loss model answers how likely additional drive failures are during degraded exposure. Adding the two into one score can help rank scenarios, but that score is not itself a calibrated probability of data loss.

RAID reliability covers only part of storage resilience. Backups, restore testing, checksums, scrubbing, spare logistics, enclosure design, controller redundancy, firmware practice, monitoring, and geographic copies address failure modes that an MTTDL equation cannot.

How to Use This Tool:

Use presets as starting points, then replace them with measurements from the actual storage fleet.

Choose an Environment preset or Custom, then confirm the RAID layout, drives per group, repeated groups, and decimal drive capacity.
Enter observed drive AFR, a representative full rebuild duration, and the planning horizon. Prefer fleet replacement records and rebuild tests over a nominal MTTF label.
Add repair-start delay, staged spares, spare activation time, rebuild buffer, and a correlation factor. These settings determine how long the array stays exposed and whether every group receives the same response delay.
Choose whether URE pressure contributes to the risk score, then set the URE specification and patrol-read interval. Read Reliability breakdown for the selected case and Mitigation ladder for the controlled faster-rebuild and alternative-layout comparisons.

Interpreting Results:

Start with annual fleet loss and horizon fleet loss. They answer how the modeled per-group hazard compounds across every repeated group and across the selected years. Fleet MTTDL is the reciprocal hazard expressed as time; use it as a relative comparison rather than a lifespan promise.

A longer Exposure means more time for another failure to overlap the degraded state.
URE per rebuild can be high for a large read volume even when overlap-based annual loss remains low. Keep the mechanisms separate when diagnosing the result.
The risk band uses a composite score. It changes when URE inclusion is toggled even though annual fleet loss and MTTDL do not.
The mitigation comparison holds most assumptions fixed. A lower modeled probability is a reason to investigate an option, not proof that capacity, performance, cost, or restore objectives will also improve.

Technical Details:

This is a documented comparative heuristic. It converts AFR to a constant hourly hazard, estimates overlapping failures during degraded exposure, scales that hazard for repeated groups, and calculates URE pressure from a rebuild-sized bit count. Full precision is retained until values are formatted for display.

Formula Core

AFR is converted from a one-year probability to a constant hourly hazard rather than divided directly by 8,760.

λ = \frac{- \ln (1 - \frac{AFR}{100})}{8760}

Buffered rebuild time and the weighted repair delay form the exposure window. At most one staged spare is credited to each repeated group.

E = R (1 + \frac{B}{100}) + \frac{c A + (G - c) D}{G} where c = \min (S, G)

R is base rebuild hours, B is rebuild buffer percent, S is staged spares, G is group count, A is spare activation hours, and D is repair-start delay.

Mechanism Core

Each layout uses a different overlap expression. The resulting raw hazard is multiplied by the correlation factor k.

RAID overlap hazard formulas
Layout	Group hazard before correlation	Drive-count rule
RAID 1	λ²E	Exactly 2 drives
RAID 5	C(n, 2)λ²E	At least 3 drives
RAID 6	C(n, 3)λ³E²	At least 4 drives
RAID 10	(n / 2)λ²E	Even count of at least 4

{MTTDL}_{group} = \frac{1}{k h} {MTTDL}_{fleet} = \frac{1}{k h G}

The same hazard is converted to annual and horizon probabilities without rounding between stages.

p_{group} = 1 - e^{- k h \times 8760} p_{fleet} = 1 - {(1 - p_{group})}^{G} p_{horizon} = 1 - {(1 - p_{fleet})}^{Y}

URE and risk-score rules

Rebuild read volume uses decimal terabytes. Mirrors read one drive-equivalent; parity layouts read at least n minus 1 drive-equivalents. With bit error rate q and b bits read, raw URE pressure is:

p_{URE} = 1 - {(1 - q)}^{b}

A patrol-read multiplier of square root of interval days divided by 30 is bounded from 0.7 to 1.2 and applied to that URE pressure. Annual URE pressure uses expected rebuild events across the fleet. When URE inclusion is enabled, the risk score adds annual fleet loss to weighted annual URE pressure, multiplies by 100, and bounds the result from 0% to 100%. The weights are 1 for RAID 5, 0.6 for RAID 1, 0.45 for RAID 10, and 0.25 for RAID 6.

Composite modeled risk bands
Risk score	Band
< 0.1%	Low modeled risk
≥ 0.1% and < 0.8%	Moderate modeled risk
≥ 0.8% and < 3%	Elevated modeled risk
≥ 3%	High modeled risk

Limitations:

The calculation is intentionally comparative and should not be used as a warranty, certification, or sole basis for a storage design.

Constant independent drive hazards are only a baseline. Field failures vary with age and show correlation.
The correlation factor is a user-selected multiplier, not a fitted model of a specific fleet.
URE scoring is a separate planning pressure and may overlap with failure consequences; the composite score is not a true loss probability.
The planned refresh cycle is recorded for audit context but does not change the constant annual probability.
Controller, firmware, filesystem, enclosure, power, site, operator, backup, and restore failures are outside the model.

Worked Examples:

Eight-drive RAID 6 groups

Four RAID 6 groups with eight 16 TB drives each, 1.5% AFR, a 24-hour rebuild, no delay, no buffer, and correlation factor 1 produce about 1.51 trillion hours of fleet MTTDL in the overlap heuristic. The annual fleet loss is about 5.8 × 10^-9. The same scenario shows substantial URE pressure at 1 in 10¹⁵ bits, but that pressure does not change MTTDL unless it is deliberately blended into the separate risk score.

References:

Disk Failures in the Real World: What Does an MTTF of 1,000,000 Hours Mean to You?, USENIX Association, February 2007.
A Case for Redundant Arrays of Inexpensive Disks, UC Berkeley EECS, 1987.
How to check disk health in Linux, Simplified Guide.