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ADR-056: Three fixed layout strategies with tunable parameters

Date: 2026-02-21 Status: Accepted

Context

Pre-sales engineers need datastore layout recommendations but have different priorities depending on the customer conversation:

  • Some customers prioritize cost (fewest datastores, lowest management overhead)
  • Some prioritize performance (workload isolation, latency guarantees)
  • Some prioritize operational simplicity (uniform sizing, predictable management)

A single "best" layout cannot satisfy all three goals simultaneously.

Decision

Generate three alternative layouts in parallel, each optimized for a different priority. The user sees a side-by-side comparison and picks the one that fits their customer's priorities.

Strategy 1: Consolidation

  • Goal: Minimize datastore count
  • Algorithm: Multi-dimensional BFD packing VMs as tightly as possible
  • Trade-off: Higher contention risk, coarser snapshot granularity

Strategy 2: Performance

  • Goal: Maximize workload isolation and minimize I/O contention
  • Algorithm: Two-phase placement — classify VMs into Hot/Warm/Cold tiers, then BFD each tier separately with tier-specific constraints
  • Rules: Hot tier (databases, >500 IOPS) limited to 15 VMs/DS; anti-affinity prevents co-locating Database and VDI workloads
  • Trade-off: More datastores, lower capacity utilization

Strategy 3: Uniform

  • Goal: All datastores same size, balanced utilization
  • Algorithm: LPT (Longest Processing Time) balanced assignment across a pre-computed number of equal-sized datastores
  • Trade-off: Doesn't adapt to workload skew, may waste space on light DS

Tunable parameters (Advanced Settings panel)

All three strategies share the same configurable constraints:

  • Max datastore capacity (default: 4 TB — Dell best practice sweet spot)
  • Max VMs per datastore (default: 25 — Dell recommendation)
  • IOPS budget per datastore (default: 100,000)
  • Snapshot reserve % (default: 15%)
  • Growth margin % (default: 20%)

Consequences

  • The user always sees three options — no single "magic answer" that may be wrong for their specific customer context.
  • Comparison metrics (DS count, utilization, isolation score, snapshot granularity) help the user explain the trade-offs to customers.
  • Default values are sourced from Dell's VMware vSphere Best Practices (H18116) and validated by domain research. Pre-sales engineers can override via the Advanced Settings panel.
  • Adding a fourth strategy in future is straightforward: implement a new function and add it to generate_all_proposals().