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The description shown below is the top answer as voted by the Spotlight community.
The charts are based on 2.1 TB of data collected from 4207 instances uploaded by 323 Spotlight users over an 8 week period.
This is an idle wait and so it can be safely ignored.
This wait has never occurred recently
PWAIT_REPLICA_ONLINE_INIT_MUTEX has not occurred recently on 4207 instances checked.