You may notice On-Demand (OD) instances launching even when the Spot instance allocation is configured at 100% in Elastigroup. This behavior is intentional and occurs under certain conditions related to cost optimization and workload stability.

Setting Spot allocation to 100% instructs Elastigroup to prefer Spot instances; however, the algorithm evaluates multiple factors before finalizing instance launches. Spot instances may not always be selected in the following scenarios:

Savings threshold not met

• Elastigroup requires a minimum Spot savings compared to On-Demand price before opting for Spot.

• By default, this threshold is 20%. If the available Spot markets offer less than 20% savings, Elastigroup launches OD instances.

  • Example: If Spot prices fluctuate closer to OD cost, stability is prioritized and OD is used.

Market availability and interruption risk

• If Spot markets indicate higher risk of interruption (or insufficient stability), Elastigroup prioritizes availability by launching OD instances.

• Applies heavily when using Availability-oriented cluster orientation, where continuity is a priority.

Restricted instance type options

• Configuring a small or narrow whitelist of Spot instance types can reduce viable Spot markets.

• When not enough suitable Spot markets exist, the algorithm defaults to OD to maintain required capacity.

Risk and cost optimization filters

• Elastigroup dynamically applies complex filters to ensure capacity, cost optimization, and system safety.

• This means OD launch is a proactive fallback under certain pricing or operational risk conditions.

Key points to remember

• Setting Spot to 100% represents preference, not an absolute rule.

• Cost savings (default 20%) and stability thresholds influence the final decision.

• OD is utilized as a fallback mechanism to ensure workload continuity and scalability.

If needed, admins can request updating the savings percentage threshold in internal configurations to alter fallback behavior.

Reference: Cluster orientation and Spot allocation logic