RAID Planning Suite

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RAID level:

Compare standard stripe, mirror, parity, and nested-parity layouts under one plan.

Installed drives:

Use the planned bay population, including any dedicated spares.

drives

Drive size:

The model assumes equal-sized members; mixed-drive stranding belongs in the dedicated capacity calculator.

Hot spares:

Set zero when the design has no dedicated standby member.

drives

Parity groups:

Each RAID 50 group needs at least three active drives; RAID 60 needs four.

groups

Drive profile:

Replace profile estimates with measured values in a detailed performance or reliability review before purchase.

Workload profile:

Choose the closest steady-state pattern; the result remains a planning estimate, not a benchmark.

Target usable capacity:

Use the capacity the workload must actually consume, before future-growth assumptions.

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Target fill:

Lower this only when the platform needs explicit operating headroom.

Filesystem overhead:

Use a platform-specific measured or documented value when available.

Annual data growth:

The action ledger shows whether the current plan covers year three.

Rebuild contention:

Use a conservative sustained contention estimate for busy arrays.

Performance safety margin:

Apply only an intentional operations or uncertainty holdback.

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A RAID layout can meet a capacity target and still be a poor fit for its workload or recovery window. Drive count and size determine how much storage exists, but parity geometry changes write service, media speed changes rebuild time, and a long rebuild leaves the array exposed to another failure or an unreadable sector. A credible plan evaluates those effects together.

The physical roles come first. Dedicated hot spares are installed but inactive. The remaining drives are assigned to data, mirrors, or parity, and nested layouts may leave drives idle when the active count does not divide evenly into groups. Filesystem overhead and the chosen target-fill percentage then reduce protected capacity to the amount planned for use.

Capacity fit asks whether planned usable bytes meet today's target and its compounded three-year growth case.
Workload fit compares modeled mixed input/output operations per second (IOPS) and sequential throughput with a selected workload profile.
Recovery exposure estimates rebuild hours, unrecoverable read error pressure, and additional-drive-failure pressure during that window.
Planning pressure combines shortfalls into one comparison score without replacing the underlying capacity, performance, and recovery results.

Parity and mirrors spend resources differently. RAID 5 and RAID 6 retain more data capacity than RAID 10 for many drive counts, but small writes carry a parity penalty unless they become full-stripe writes. RAID 10 gives up half its engaged capacity to mirrors and may offer stronger random-write service. RAID 50 and RAID 60 distribute parity across groups, making group count part of both capacity and failure-domain planning.

Drive profiles and workload profiles are scenario assumptions, not measurements of a particular shelf. Controller cache, queue depth, stripe size, firmware, filesystem behavior, background scrubs, and degraded operation can move real results far from a simple model. Replace a profile assumption with benchmark evidence in the final design review, even when the calculated plan is comfortable.

RAID is an availability mechanism for selected member failures. It is not a backup and does not cover deletion, ransomware, silent corruption outside the protection path, controller loss, or a site incident. Recovery planning still needs independent copies and tested restore procedures.

How to Use This Tool:

Resolve the array geometry first, then add workload and recovery assumptions without treating preset profiles as measured guarantees.

Choose the RAID level and enter installed drives, equal effective drive size, unit, and dedicated hot spares. For RAID 50 or RAID 60, set two to 16 parity groups.
Select the closest drive and workload profiles. The profile values provide comparative IOPS, throughput, rebuild, annual failure, and read-error assumptions.
Enter the target usable capacity and target fill. Add filesystem overhead and annual growth when the planned pool must preserve operating room or future runway.
Apply rebuild contention and a performance safety margin when real workloads reduce replacement speed or service rates. These percentages lower the relevant rates before results are compared.
Read the capacity gap, IOPS and throughput headroom, rebuild window, combined recovery pressure, and action list. Compare layouts under the same inputs before choosing a candidate for controller validation and benchmarking.

Interpreting Results:

Planned usable capacity applies filesystem overhead and target fill to the selected layout's data-drive equivalents. A target can be met while the status still reports a workload shortfall because capacity, IOPS, and throughput are tested separately.

RAID planning result priorities
Status	Condition	What to inspect
Capacity target is short	Planned usable bytes are below the current target.	Data-drive count, fill, overhead, member size, and target unit.
Workload target is short	Capacity passes, but IOPS or throughput headroom is negative.	Media profile, workload mix, parity penalty, safety margin, and benchmarks.
Recovery pressure needs review	Capacity and performance pass, but planning pressure is at least 50%.	Rebuild hours, redundancy, URE input, and additional-failure pressure.
Planning capacity	The preceding status gates pass.	Controller support, degraded behavior, backup, and restore objectives still need proof.

Headroom above zero means modeled service exceeds the workload profile's target; negative headroom is a shortfall. The pressure score is bounded from zero to 100% and is useful for comparing the same assumptions across layouts. It is not a probability of data loss.

A comparison row that looks best on pressure may still violate a non-modeled requirement such as controller support, write latency, failure-domain policy, or rebuild priority. Use the action list as a review order, then validate the chosen design with the actual platform.

Technical Details:

The model resolves RAID geometry before calculating capacity or service. Active drives equal installed drives minus spares. RAID 10 engages complete pairs; RAID 50 and RAID 60 engage complete equal-width groups. Any remaining active drives are idle and contribute neither data nor protection capacity.

Formula Core:

Capacity, mixed service, rebuild exposure, recovery pressure, and growth all use the same resolved geometry and selected profile values.

\begin{matrix} C_{plan} & = & N_{data} S (1 - \frac{o}{100}) \frac{f}{100} \\ H (x, y, r) & = & \frac{1}{\frac{r}{x} + \frac{1 - r}{y}} \\ I_{mixed} & = & H (I_{read}, I_{write}, r) (1 - \frac{m}{100}) \\ t_{rebuild} & = & \frac{S}{V (1 - \frac{c}{100}) 1000^{2} 3600} \\ P_{URE} & = & 1 - e^{- \frac{B_{read}}{B_{perURE}}} \\ C_{3} & = & C_{target} {(1 + \frac{g}{100})}^{3} \end{matrix}

N_data is data-drive equivalents, S is member bytes, o is filesystem overhead percent, and f is target fill percent. The harmonic blend H combines read and write service using read share r; performance margin m reduces the result. Rebuild speed V is reduced by contention c. Three-year target C₃ compounds annual growth g.

Mechanism Core:

Read IOPS scale with engaged drives. Write IOPS use one service drive for RAID 1, the data-drive count for RAID 10, all engaged drives for RAID 0, or engaged drives divided by an effective parity penalty. The base write penalties are 1 for RAID 0, 2 for RAID 1 and RAID 10, 4 for RAID 5 and RAID 50, and 6 for RAID 6 and RAID 60. For parity layouts, full-stripe writes reduce the effective penalty toward 1.

Sequential throughput uses the same harmonic blend with profile MB/s rates. Parity write service combines penalized writes and full-stripe writes; the full-stripe share uses all data-drive equivalents. IOPS and throughput headroom are each calculated as modeled service minus profile target, divided by that target.

Rebuild reads are modeled as the surviving engaged member count multiplied by member bits. Raw URE pressure follows the exponential equation above; layouts with two-failure tolerance multiply that raw value by 0.1. Additional-failure pressure uses a Poisson model with annual failure rate, exposed drives, and rebuild hours, with a 1.1 multiplier when at least 12 drives are engaged. The two pressures are combined as the probability of either event.

P_{score} = 0.35 P_{capacity} + 0.25 P_{performance} + 0.20 P_{rebuild} + 0.20 P_{protection}

Capacity pressure is the percentage target shortfall, clamped from 0 to 100. Performance pressure averages clamped negative IOPS and throughput headroom. Rebuild pressure is rebuild hours divided by 48 hours, expressed as a clamped percentage. Protection pressure is 100 for RAID 0 and otherwise the clamped combined recovery pressure.

Lookup Core:

Preset values are explicit scenario inputs to the formulas. They are not detected from hardware.

Drive profile constants
Drive profile	Read / write IOPS	Read / write MB/s	Rebuild MB/s	AFR	Bits per URE
NAS HDD	190 / 170	245 / 230	210	1.8%	10¹⁴
Enterprise HDD	240 / 220	285 / 265	275	1.4%	10¹⁵
SATA SSD	85,000 / 42,000	540 / 500	850	0.8%	10¹⁶
NVMe SSD	650,000 / 240,000	6,200 / 4,600	2,400	0.7%	10¹⁶

Workload profile constants
Workload	Reads	Full-stripe writes	Target IOPS	Target MB/s
File services	70%	35%	45,000	1,600
Virtualization	68%	12%	220,000	2,600
Database / OLTP	62%	4%	320,000	1,800
Backup / archive	35%	82%	18,000	4,200
Analytics / read heavy	88%	45%	160,000	5,200

Rule Core:

RAID 0, 1, 5, 6, 10, 50, and 60 require at least 2, 2, 3, 4, 4, 6, and 8 active drives respectively. RAID 10 uses complete pairs. RAID 50 and RAID 60 require two to 16 groups with at least three or four active drives per group. Invalid geometry withholds the plan.

Action thresholds are independent of the headline status: rebuild at or above 36 hours requests a shorter recovery window, combined recovery pressure at or above 5% requests review, any three-year gap requests growth planning, and RAID 0 requests redundancy. If no action threshold is reached, the plan still asks for controller, benchmark, backup, and restore validation.

Limitations and Accuracy Notes:

The calculations are comparative estimates built from fixed profiles and simplified independence assumptions. They do not inspect a controller, SMART data, queue depth, firmware, cache policy, or degraded-mode measurements.

Annual failure rate and bits-per-URE are population or specification inputs, not predictions for one drive.
The Poisson failure model cannot represent shared firmware defects, enclosure faults, vibration, temperature, or other correlated causes beyond its one width multiplier.
Controller coalescing, stripe size, caching, filesystem behavior, and workload burstiness can change real IOPS and throughput.
Validate rebuild time under representative contention and confirm the restore objective separately.
The entered values are evaluated in the browser and are not sent to a dedicated planning service.

Worked Examples:

Capacity passes while enterprise HDD IOPS fail

Twelve 18 TB drives with one spare leave 11 active members. RAID 6 assigns nine to data and two to protection, producing 162 TB before fill deductions. Against a 100 TiB file-services target, capacity passes, but the enterprise-HDD profile models about 1,263 mixed IOPS against a 45,000 IOPS target, so the headline status is a workload shortfall.

SATA SSD mirrors clear the selected targets

Eight 4 TB SATA SSDs in RAID 10 provide four data-drive equivalents and 16 TB planned capacity with no deductions. Against a 14 TB virtualization target, the model shows 2 TB surplus, about 344,262 mixed IOPS, roughly 3,151 MB/s, and a 1.31-hour rebuild estimate under the preset assumptions.

References:

RAID level summary, IBM Documentation.
Redundant Array of Independent Devices, Storage Networking Industry Association Dictionary.
Exos X20 Product Manual, Seagate.