Impact of changing external labels

[svend]

What is the impact to Thanos when a Prometheus instance's external labels are changed? For example, I'd like to add a new region label to a Prometheus instance. It seems like it could impact compaction and downsampling, since the instance uniqueness is identified by the external labels that on a block when it was written to object storage. I'm also wondering how it would affect Thanos queries.

There is a related related GitHub issue that was closed, so I'd like to continue the discussion here.

Has anyone changed external labels, and what was your experience?