The migration threshold specifies the tolerance of imbalance of the Current Host Load Standard Deviation relating to the Target Host Load Standard Deviation. DRS assign a priority level to a recommendation and compared to the migration threshold. If the priority level is less than or equal to the migration threshold, the recommendation is displayed or applied.
Level 1 (Conservative)
When Selecting this level, only mandatory moves, priority-one recommendations are executed. Mandatory moves are issued when:
- The ESXi host enters maintenance mode, Host enters standby mode, An affinity or anti-affinity rule is violated, The sum of the reservations of the VMs exceeds the capacity of the host
Level 2 (moderately conservative)
The level 2-migration threshold only applies priority-one and priority-two recommendations, priority two recommendations promise a very good improvement in the cluster’s load balance.
Level 3 (moderate)
The level 3-migration threshold is the default migration threshold when creating DRS clusters. The moderate migration threshold applies priority-one, two and three recommendations, promising a good improvement in the cluster’s load balance.
Level 4 (moderately aggressive)
The level 4-migration threshold applies all recommendations up to priority level four. Priority-four recommendations promise a moderate improvement in the cluster’s load balance.
Level 5 (aggressive)
The level 5 migration threshold is the right-most setting on the migration threshold slider and applies all five priority level recommendations; every recommendation which promises even a slight improvement in the cluster’s load balance is applied.
VMware DRS assigns integer priority ratings to the migration recommendations it makes. You can better understand the meaning of a priority rating if you are aware of the algorithm that calculates it.
Source this information is VMware vSphere 5 Clustering technical deepdive from Frank Denneman and Duncan Epping, Thanks to them and VMware.
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