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coordinator processes also modify metadata. Queries can be performed

One more please, do I need to backup zookeeper?

I think no need for zookeeper. It only holds actual cluster state

This is as per guide: To ensure a clean migration, shut down the non-coordinator services to ensure that metadata state will not change as you do the migration. https://druid.apache.org/docs/latest/operations/metadata-migration/

This requirements is only applicable for built-in Derby metadata I suppose