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ADR-058: Dedicated datastore for mission-critical VMs

Date: 2026-02-21 Status: Accepted

Context

The Performance strategy uses BFD to pack VMs into datastores with tier-specific density caps (10–25 VMs/DS). However, certain mission-critical workloads should never share a datastore with other VMs:

  • SAP HANA: Multi-TB memory footprint, extreme IOPS, requires dedicated snapshot/backup windows. Best practice is separate volumes for data/log/shared.
  • Exchange: Latency-sensitive mailbox I/O, large databases, requires isolated backup granularity for DAG recovery.
  • Large Oracle RAC: Similar profile — dedicated QoS, independent snapshot scope.

Placing a SAP HANA VM alongside 9 other VMs on a "Hot tier" datastore defeats the purpose of workload isolation:

  • PowerStore volume-level snapshots would capture all 10 VMs, not just HANA
  • QoS policy applies to the shared volume, not just HANA
  • Restore requires mounting the entire 10-VM volume

Decision

Add a Phase 0 isolation pass to the Performance strategy: before BFD placement begins, extract VMs that match isolation criteria and assign each to a dedicated 1:1 datastore. Remaining VMs proceed through normal Hot/Warm/Cold tiered BFD.

Isolation triggers (any one is sufficient):

  1. Workload category contains "SAP HANA" or "Exchange"
  2. VM provisioned capacity exceeds 2 TB
  3. VM IOPS exceeds 5,000

These thresholds are configurable via the Advanced Settings panel. The isolation list can be extended by adding workload category patterns.

Isolated datastores are sized to the individual VM (with growth margin and snapshot reserve applied), not to the global max datastore size. A 3 TB HANA VM gets a ~4.6 TB datastore (3 TB / 0.65 usable ratio), not a 4 TB standard DS.

Consequences

  • The Performance strategy now has three phases: isolate → classify tiers → BFD per tier. This matches how storage architects actually design layouts.
  • Isolated VMs contribute to total datastore count and raw capacity metrics but have utilization close to 100% and isolation score = 1.0.
  • The Consolidation and Uniform strategies do not apply isolation — by design, they prioritize density and balance respectively. A user who wants isolation should choose the Performance strategy.
  • The comparison table naturally shows the impact: Performance strategy will have more datastores but better isolation score and finer snapshot granularity.