What-If Capacity Modeling

Invoke-S2DCapacityWhatIf models the capacity impact of proposed hardware changes — adding nodes, adding disks, replacing disks, or changing resiliency — without touching the live cluster.

It applies scenario modifications to a baseline capacity waterfall and recomputes all 7 stages, producing a side-by-side before/after comparison with per-stage deltas.

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Syntax

# From a JSON snapshot
Invoke-S2DCapacityWhatIf -BaselineSnapshot <path> [scenario parameters] [-OutputDirectory <path>] [-Format <Html|Json>] [-PassThru]

# From a piped S2DClusterData object
Invoke-S2DCartographer -ClusterName <name> -PassThru |
    Invoke-S2DCapacityWhatIf [scenario parameters]

# Against a live cluster (collects then models — does not re-hit the cluster on repeated runs)
Invoke-S2DCapacityWhatIf -ClusterName <name> [scenario parameters]

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Parameters

Baseline input (choose one)

Parameter	Description
-BaselineSnapshot	Path to a JSON snapshot produced by Invoke-S2DCartographer (SchemaVersion 1.0).
-InputObject	An S2DClusterData object piped from Invoke-S2DCartographer -PassThru.
-ClusterName	Connect to a live cluster, collect data, then model. Does not re-collect on repeated -PassThru calls.

Scenario modifications (combinable)

Parameter	Description
-AddNodes <int>	Number of nodes to add. New nodes are assumed to have the same disk configuration as existing nodes (average disk count × disk size).
-AddDisksPerNode <int>	Number of capacity disks to add per node. Enforces symmetry — all existing nodes receive the same count.
-NewDiskSizeTB <double>	Size in decimal TB for new disks added via -AddNodes or -AddDisksPerNode. Defaults to the existing largest disk.
-ReplaceDiskSizeTB <double>	Model replacing all capacity disks with disks of this size in decimal TB. Node count and disk count remain the same.
-ChangeResiliency <int>	Override the resiliency factor (NumberOfDataCopies). 2 = 2-way mirror, 3 = 3-way mirror.

Scenario parameters can be combined in a single invocation for a composite what-if.

Output

Parameter	Description
-OutputDirectory <string>	Directory to write reports to. If omitted, no files are written.
-Format <string[]>	Report formats: Html (default), Json, or both.
-PassThru	Return the S2DWhatIfResult object to the pipeline.

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Output object — S2DWhatIfResult

S2DWhatIfResult
├── ScenarioLabel          string          Human-readable description of applied changes
├── BaselineNodeCount      int             Node count before the scenario
├── ProjectedNodeCount     int             Node count after the scenario
├── DeltaUsableTiB         double          Net change in usable capacity (TiB)
├── DeltaUsableTB          double          Net change in usable capacity (TB)
├── GeneratedAt            string          ISO 8601 timestamp
├── BaselineWaterfall      S2DCapacityWaterfall
├── ProjectedWaterfall     S2DCapacityWaterfall
└── DeltaStages[]
    ├── Stage              int             Stage number (1–8)
    ├── Name               string          Stage name
    ├── BaselineTiB        double          Baseline size at this stage
    ├── ProjectedTiB       double          Projected size at this stage
    ├── DeltaTiB           double          Difference (positive = gain)
    ├── BaselineTB         double
    ├── ProjectedTB        double
    └── DeltaTB            double

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Examples

Add 2 nodes

Model scaling out from 4 to 6 nodes on a cluster with 4 × 3.84 TB drives per node:

Invoke-S2DCapacityWhatIf -BaselineSnapshot C:\snapshots\clus01-20260101.json -AddNodes 2

The projected waterfall recomputes all 7 stages with 6 nodes × 4 disks. Reserve deduction increases (min(6,4) × largest drive remains capped at 4), but raw capacity grows proportionally.

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Add 4 disks per node (same disk size)

Invoke-S2DCapacityWhatIf `
    -BaselineSnapshot C:\snapshots\clus01-20260101.json `
    -AddDisksPerNode 4

Disk count per node grows from 4 to 8. The new disks are assumed to be the same size as the existing largest disk. Raw bytes and all downstream stages increase proportionally.

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Add disks with a specific size

When adding drives of a different size (e.g., upgrading to 7.68 TB NVMe):

Invoke-S2DCapacityWhatIf `
    -BaselineSnapshot C:\snapshots\clus01-20260101.json `
    -AddDisksPerNode 4 `
    -NewDiskSizeTB 7.68

The projected largest-disk size used for reserve calculation will be max(existing, new).

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Replace all capacity disks

Model replacing 3.84 TB drives with 7.68 TB drives across all nodes (same node and disk count):

Invoke-S2DCapacityWhatIf `
    -BaselineSnapshot C:\snapshots\clus01-20260101.json `
    -ReplaceDiskSizeTB 7.68

Total disk count is unchanged. All stages recompute using the new drive size as both the per-disk size and the largest-disk reserve multiplier.

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Change resiliency (3-way → 2-way mirror)

Invoke-S2DCapacityWhatIf `
    -BaselineSnapshot C:\snapshots\clus01-20260101.json `
    -ChangeResiliency 2

Stage 7 (Usable Capacity) changes. All earlier stages are unchanged because resiliency does not affect pool or reserve math. Blended efficiency changes from 33.3% to 50.0%.

WARNING

Resiliency change is a cluster-level operation Changing resiliency on a live cluster requires draining, stopping, and reconfiguring volumes. This cmdlet models the capacity outcome only — it does not make any live changes.

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Composite: add 2 nodes + more disks + new resiliency

Invoke-S2DCapacityWhatIf `
    -BaselineSnapshot C:\snapshots\clus01-20260101.json `
    -AddNodes 2 `
    -AddDisksPerNode 4 `
    -NewDiskSizeTB 7.68 `
    -ChangeResiliency 2

All four modifications are applied together. The ScenarioLabel property lists each change: +2 nodes, +4 disks/node, Resiliency → 2-way mirror.

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Pipeline from live run

Run the full audit on a cluster and immediately feed the result into what-if modeling:

$data = Invoke-S2DCartographer -ClusterName clus01 -PassThru
$whatif = $data | Invoke-S2DCapacityWhatIf -AddNodes 2 -PassThru
$whatif.DeltaUsableTiB

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Generate HTML and JSON reports

Invoke-S2DCapacityWhatIf `
    -BaselineSnapshot C:\snapshots\clus01-20260101.json `
    -AddNodes 2 `
    -AddDisksPerNode 4 `
    -OutputDirectory C:\Reports\WhatIf `
    -Format Html, Json

Writes two files to C:\Reports\WhatIf\:

• whatif-20260101-1430.html — interactive before/after waterfall chart with KPI summary and delta table
• whatif-20260101-1430.json — structured JSON with full baseline and projected waterfall data

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How the model works

Baseline extraction

When loading from a JSON snapshot, the cmdlet extracts:

• Node count — from NodeCount field, or counted from unique node names on capacity disks
• Disks per node — total capacity disks ÷ node count (averaged, rounded)
• Largest disk size — maximum SizeBytes across pool-member capacity-tier disks
• Pool total / free bytes — from StoragePool.TotalSize and RemainingSize
• Infrastructure volume bytes — from FootprintOnPool (or Size) of volumes where IsInfrastructureVolume = true
• Resiliency factor — from StoragePool.ResiliencySettings Mirror entry NumberOfDataCopies; defaults to 3.0

Projection calculation

Scenario parameters modify the extracted baseline inputs:

1. -ReplaceDiskSizeTB — overrides the per-disk size used for raw byte calculation
2. -NewDiskSizeTB — sets the size for newly added disks; projected largest-disk = max(existing, new)
3. -AddDisksPerNode — increments disks-per-node count
4. -AddNodes — increments node count
5. -ChangeResiliency — overrides the resiliency factor

Projected raw bytes = projected node count × projected disks per node × projected disk size

Projected pool = estimated as projected raw × 99% (standard pool overhead).

Infrastructure volume bytes are unchanged — the model assumes existing infra volumes are unchanged regardless of hardware changes.

The same Invoke-S2DWaterfallCalculation pure function computes both the baseline and projected waterfall from these inputs.

Limitations

• New nodes are assumed to be symmetric — same disk count and size as the model inputs. If your real nodes will have a different configuration, chain -AddNodes and -ReplaceDiskSizeTB together.
• The model does not account for cluster rebuild time, performance impact, or rolling upgrade constraints.
• Thin-provisioned volume growth risk is modeled by Get-S2DHealthStatus (checks 6 and 11) — not by this cmdlet.

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Related

• Capacity waterfall — how the 7 stages are computed
• Capacity math — reserve calculation, TiB vs TB, efficiency
• Health checks — thin provisioning risk (checks 6 and 11)
• JSON snapshot schema — baseline snapshot format