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It'll show availability of 100% if there is at least 1 replica loaded for all segments. By default replication happens pretty slowly in the background. You can modify the coordinator dynamic configs to speed up replica loads as the default is to do 10 replica loads per historical per coordinator run. You can use the segment load API to chech the status of the replication while rolling the nodes (https://druid.apache.org/docs/latest/operations/api-reference.html#segment-loading) so you can be sure it's fully replicated beforehand.