RAID Planning Suite

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RAID layout:

Choose the target layout before comparing capacity, IOPS, rebuild exposure, and sizing runway.

Nested parity groups:

Only affects RAID 50 and RAID 60. Incomplete groups leave drives idle in the capacity ledger.

groups

Installed drives:

Use the planned bay population, not only data drives; hot spares are counted separately below.

drives

Drive size:

Uniform-drive planning keeps the merged suite fast; use the source capacity calculator for mixed-disk shelves.

Hot spares:

Enter 0 when the array has no dedicated spare drive.

drives

Drive profile:

Use Custom in Advanced when you have measured drive IOPS, rebuild speed, or annualized failure rate.

Workload profile:

Pick the closest steady-state workload, then tune read ratio or targets in Advanced if needed.

Target fill: {{ formatPercent(target_fill_pct / 100, 0) }}

80-85% is a common operations target for shared arrays with snapshots or churn.

Target usable:

Set the capacity you need after reserves; RAID Growth Path projects future drive counts from this starting point.

Annual growth: {{ formatPercent(annual_growth_pct / 100, 0) }}

Use 0% for a fixed target or set expected yearly data growth.

Read ratio: {{ formatPercent(read_ratio_pct / 100, 0) }}

Higher read ratios favor RAID 5/6 capacity layouts; write-heavy mixes surface parity penalty faster.

Full-stripe writes: {{ formatPercent(full_stripe_write_pct / 100, 0) }}

Leave low for random databases; raise for backup, media, or large sequential ingest.

Target IOPS:

Enter 0 to skip IOPS target pressure in the summary.

Target throughput:

Use MB/s from benchmark goals, backup windows, or application requirements.

MB/s

Per-drive random IOPS:

Keep the profile values unless you have measured drive-level IOPS.

Per-drive sequential speed:

First value is read MB/s, second value is write MB/s.

MB/s

Rebuild rate:

Use a conservative sustained rate, especially for busy parity arrays.

MB/s

Rebuild contention: {{ formatPercent(rebuild_contention_pct / 100, 0) }}

Higher contention means longer degraded exposure and lower rebuild headroom.

Annual failure rate:

Profile defaults are conservative planning values, not vendor guarantees.

URE rate:

10^-15 is common for enterprise media; 10^-14 is a more pessimistic consumer-disk assumption.

Filesystem overhead: {{ formatPercent(filesystem_overhead_pct / 100, 1) }}

Set 0% for raw RAID math or keep a small platform reserve for planning.

Per-drive reserve:

Leave 0 when your storage platform already reports usable member size.

GB/drive

Area	Metric	Value	Copy
{{ row.area }}	{{ row.metric }}	{{ row.value }}

Layout	Usable	Efficiency	Mixed IOPS	Rebuild	Risk	Copy
{{ row.layout }}	{{ row.usable }}	{{ row.efficiency }}	{{ row.mixedIops }}	{{ row.rebuild }}	{{ row.risk }}

Priority	Signal	Detail	Copy
{{ row.priority }}	{{ row.signal }}	{{ row.detail }}

Horizon	Target usable	Current runway	Drives needed	Action	Copy
{{ row.horizon }}	{{ row.target }}	{{ row.runway }}	{{ row.drives }}	{{ row.action }}

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A storage purchase can look generous on a quote and still miss the real operating target. RAID uses several drives as one protected storage set, but the array has to spend some of those drives on parity, mirrors, spares, and unused headroom before it becomes a usable pool for applications. The right comparison is not raw terabytes against a capacity request; it is protected capacity, workload service rate, recovery exposure, and growth room under the same set of assumptions.

RAID choices also change the kind of risk the storage team accepts. Single parity keeps more usable space than mirrors, but a second failure or an unreadable sector during rebuild can matter more on large, slow disks. Dual parity reduces that pressure at the cost of more parity capacity and write work. RAID 10 trades capacity for simpler mirror recovery and stronger random-write behavior. Nested parity groups such as RAID 50 and RAID 60 add another planning question because group width affects idle drives, fault tolerance per group, and rebuild read volume.

RAID planning questions and the design choice each one affects
Question	What changes	Planning consequence
How many bays are active after spares?	Data, parity, mirror, and idle drive counts	The selected layout may become impossible or less efficient than expected.
How full should the array run?	Plannable capacity after filesystem overhead and target fill	A pool that technically fits the data may still lack snapshot, churn, or growth room.
How write-heavy is the workload?	Parity write penalty, full-stripe benefit, and mixed IOPS	Capacity-efficient parity can lose to mirrors when random writes dominate.
How long does rebuild take?	Degraded exposure, URE pressure, and approximate MTTDL	Large drives and slow recovery can make an otherwise efficient plan fragile.

RAID planning divides bays into data, parity, spare, idle, and reserve roles

Unit language is another source of mistakes. Drive vendors usually label disks in decimal TB, while storage operating systems and management consoles often report binary TiB. A plan that mixes the two can appear several percent larger or smaller than expected before RAID overhead is even applied. Filesystem metadata, per-drive reserve, and target fill then reduce the number again.

RAID improves availability for selected drive failures; it does not replace backup, replication, snapshots, or restore testing. A useful RAID plan keeps those separate: RAID helps the array stay available during a failed member, while recovery planning covers deletion, corruption, ransomware, controller failure, and site loss.

How to Use This Tool:

Work from layout geometry first, then tune workload and recovery assumptions. That order helps catch impossible drive counts before performance or growth numbers distract from the basic array shape.

Choose the RAID layout. For RAID 50 or RAID 60, set nested parity groups when the design has fixed group counts, or leave the value at zero so the planner selects a practical grouping.
Enter installed drives, drive size, capacity unit, and hot spares. Count planned bays, not only data drives, because spares and idle drives affect the final capacity ledger.
Select the drive profile and workload profile closest to the intended use. Open Advanced when measured IOPS, sequential rates, rebuild speed, AFR, URE rate, filesystem overhead, or per-drive reserve values are available.
Set target fill, target usable capacity, and annual growth. Use the target fill as an operating limit, not as the physical maximum the array could hold.
Check the topology surface for unexpected parity, spare, idle, or reserve roles before reading the detailed results.
Compare the ledger, candidate matrix, capacity allocation, tradeoff map, recovery brief, and growth path. A result is ready to share when the chosen layout is buildable, the target gap is closed or explained, and the recovery label matches the risk you are willing to accept.

Advanced Tips:

Use measured rebuild speed under normal load when possible. A lab rebuild on an idle shelf can understate degraded exposure for a production pool.
For wide parity layouts, compare at least one narrower group plan and one mirror-based plan. The best capacity row is not always the best recovery design.
Raise filesystem overhead or reserve per drive when copy-on-write filesystems, snapshots, thin provisioning, metadata reservations, or vendor appliance overhead are part of the plan.
Adjust full-stripe writes for backup and archive targets. High full-stripe percentages can improve parity-write estimates, while small random writes keep more of the read-modify-write penalty.
Use the growth path before ordering shelves. Near-term drive additions can force a new group width, controller choice, chassis size, or backup window.

Interpreting Results:

Treat plannable capacity as the headline storage budget. It has already removed spare capacity, RAID protection capacity, per-drive reserve, filesystem overhead, and the target-fill reserve. Protected usable capacity is useful for understanding the physical array, but plannable capacity is the safer number for procurement and growth checks.

RAID planning result areas and reading guidance
Result area	What to read	What to verify
RAID Plan Ledger	Raw installed capacity, protected usable capacity, plannable capacity, target gap, efficiency, mixed IOPS, rebuild hours, annual loss pressure, and approximate MTTDL.	Confirm that the same units and fill target match the storage request.
RAID Candidate Matrix	RAID 0, 1, 5, 6, 10, 50, and 60 under the same drive, workload, and recovery assumptions.	Ignore rows that are not buildable with the active drive count.
RAID Capacity Allocation	How capacity is divided across plannable space, fill headroom, reserve, parity or mirrors, spares, and idle drives.	Investigate idle drives; they often mean the group shape does not divide evenly.
RAID Tradeoff Map	Usable capacity and mixed IOPS for viable layouts, with rebuild hours and risk score in the data behind the view.	Use it to shortlist options, then read the recovery brief before choosing.
RAID Recovery Brief	Verdict, capacity runway, rebuild window, IOPS headroom, and URE pressure.	A balanced verdict still depends on the entered AFR, URE rate, drive profile, and rebuild rate.
RAID Growth Path	Current, year 1, year 2, and year 3 capacity targets after annual growth.	Check whether future targets require more bays than the selected chassis can provide.

Technical Details:

RAID geometry is resolved before capacity math. Hot spares are removed from the installed count, and the remaining active drives are assigned to data, parity, mirror, or idle roles. RAID 0 stripes all active drives. RAID 1 models one data copy with the rest as mirrors. RAID 5 and RAID 6 subtract one or two parity members. RAID 10 forms even mirror pairs. RAID 50 and RAID 60 split active drives into parity groups and can leave drives idle when the count does not divide cleanly.

Capacity, service rate, and rebuild exposure are separate checks. A design can satisfy the target capacity while missing the IOPS target, or it can pass the workload estimate while carrying a long degraded window. Keeping those checks separate makes it easier to see whether the weak point is bays, media speed, parity geometry, or recovery policy.

Formula Core

Plannable capacity starts with the data-drive count and effective member size, then removes filesystem overhead and applies the target fill fraction.

C_{plan} = N_{data} \times (S_{drive} - S_{reserve}) \times (1 - O_{fs}) \times F_{target}

Ndata is the resolved data-drive count, Sdrive is member capacity in bytes, Sreserve is the per-drive reserve, Ofs is filesystem overhead as a fraction, and Ftarget is target fill as a fraction. With 10 data drives, 18 TB members, no per-drive reserve, 3% filesystem overhead, and 80% target fill, the plannable value is 10 x 18 TB x 0.97 x 0.80 before display conversion to TiB.

RAID geometry rules used by the planner
Layout	Minimum active drives	Data-drive rule	Tolerance model
RAID 0	2	All active drives	No redundancy
RAID 1	2	One data copy	Remaining drives mirror that copy
RAID 5	3	Active drives minus 1	One failed member
RAID 6	4	Active drives minus 2	Two failed members
RAID 10	4	Half of the even active count	One per mirror pair, more only when failures land in different pairs
RAID 50 / RAID 60	6 / 8	Group count times group width minus parity per group	One or two failed members per parity group

Performance Core

Mixed IOPS uses a harmonic blend so read and write service rates are weighted by demand. Reads scale with engaged drives. Writes use a service-drive factor that reflects mirror duplication or parity write penalty, with full-stripe writes reducing the parity penalty.

I_{mixed} = \frac{1}{\frac{R_{read}}{I_{read}} + \frac{R_{write}}{I_{write}}}

RAID 5 and RAID 50 start with a random-write penalty of 4. RAID 6 and RAID 60 start with 6. RAID 1 and RAID 10 use 2, and RAID 0 uses 1. For parity layouts, the effective write penalty is 1 + ((base penalty - 1) x (1 - full-stripe fraction)). The resulting mixed IOPS and throughput are compared with the target IOPS and target throughput values.

Recovery and Risk Core

Rebuild hours use drive size, replacement write rate, and rebuild contention. URE pressure uses the number of bits read during rebuild: one drive for mirror layouts, no read pressure for RAID 0, and surviving members for parity layouts.

H_{rebuild} = \frac{S_{drive}}{V_{rebuild} \times (1 - C) \times 1000000 \times 3600}

P_{URE} = 1 - e^{- B_{read} \times r_{URE}}

Additional failure pressure during rebuild is estimated from exposed drives, AFR, rebuild hours, and a small correlation multiplier for wide, large, or HDD-based arrays. The risk score adds pressure for invalid layouts, no redundancy, long rebuilds, combined rebuild and URE pressure, missed IOPS or throughput targets, missed capacity targets, and wide RAID 5. RAID 6, RAID 60, and RAID 10 receive small recovery-credit adjustments.

Risk labels used by the RAID planning model
Risk label	Score or condition	Planning response
Not buildable	The chosen layout cannot be formed from the active drive count.	Change the RAID level, reduce spares, or add drives.
High pressure	Risk score 65 or higher.	Redesign before purchase or production use.
Watch rebuild	Risk score from 40 to below 65.	Compare dual parity, narrower groups, faster media, or a lower fill target.
Balanced	Risk score from 20 to below 40.	Confirm workload benchmarks, backup coverage, and platform behavior.
Recovery-focused	Risk score below 20.	Still validate restore paths and controller-specific rebuild behavior.

Limitations and Accuracy Notes:

The calculations use the values entered in the browser. When chart views are opened, the browser may fetch the charting library needed to draw them.
The model does not inspect a live controller, read SMART data, detect actual filesystem allocation, or measure real degraded-mode throughput.
Hardware RAID, Linux mdraid, ZFS, Storage Spaces, and storage appliances can differ in write coalescing, checksumming, patrol reads, spare behavior, and rebuild priority.
AFR, URE rate, and approximate MTTDL are planning values for comparison. They are not promises for one drive, one shelf, or one site.
RAID 0 should be reserved for scratch, cache, replicated, or disposable data because any member failure can lose the array.
RAID redundancy does not cover deletion, malware, silent corruption, controller faults, site incidents, or untested restore procedures.

Worked Examples:

Example RAID planning situations
Scenario	Useful starting comparison	Main check
Eight large NAS HDDs for shared files	RAID 6 against RAID 10	Dual-parity capacity versus mirror write behavior, rebuild hours, and URE pressure.
Twenty-four-drive archive shelf	RAID 60 with fixed groups and a RAID 6 alternative	Group width, idle drives, plannable capacity, and three-year growth runway.
SSD virtualization pool	RAID 10 against RAID 6	Mixed random IOPS, degraded IOPS, rebuild time, and target fill rather than raw TiB.
Backup target with mostly sequential writes	RAID 6, RAID 50, and RAID 60	Full-stripe write percentage, throughput target, rebuild contention, and restore-time needs.

FAQ:

Why is plannable capacity lower than protected usable capacity?

Protected usable capacity is the data space after RAID geometry and filesystem overhead. Plannable capacity applies the target fill percentage so snapshots, churn, maintenance, and growth do not consume the whole pool.

Why does RAID 5 often look efficient but risky on large drives?

RAID 5 loses only one drive to parity, but it has one failed-drive tolerance. Large members and slow rebuilds keep the array degraded longer, which raises the chance that another failure or unreadable read error matters.

Why can RAID 10 show lower capacity but better workload headroom?

RAID 10 mirrors data and stripes across pairs. That usually halves usable capacity, but random writes avoid parity read-modify-write work and rebuilds usually read from the surviving mirror member.

Should approximate MTTDL decide the final layout?

No. Approximate MTTDL is useful for comparing designs under the same assumptions. It does not cover correlated failures, firmware defects, operator mistakes, environmental issues, or restore failure.

Glossary:

AFR: Annualized failure rate, used as a drive failure probability input.
Full-stripe write: A write pattern that updates a whole parity stripe and can reduce parity read-modify-write cost.
MTTDL: Mean time to data loss, an approximate comparative reliability value derived from modeled loss probability.
Plannable capacity: Protected usable capacity after the target fill percentage is applied.
URE: Unrecoverable read error, a drive read failure that can matter during rebuild because surviving members must be read to reconstruct data.

RAID Planning Suite

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Current result

How to Use This Tool:

Advanced Tips:

Interpreting Results:

Technical Details:

Formula Core

Performance Core

Recovery and Risk Core

Limitations and Accuracy Notes:

Worked Examples:

FAQ:

Glossary:

References: