SSTables streamed during the repair process will first be written locally and afterwards either simply added to the pool of existing sstables or, in case of existing MVs or active CDC, replayed on mutation basis:
As described in StreamReceiveTask.OnCompletionRunnable:
We have a special path for views and for CDC.
For views, since the view requires cleaning up any pre-existing state, we must put all partitions through the same write path as normal mutations. This also ensures any 2is are also updated.
For CDC-enabled tables, we want to ensure that the mutations are run through the CommitLog so they can be archived by the CDC process on discard.
Using the regular write path turns out to be an issue for incremental repairs, as we loose the repaired_at state in the process. Eventually the streamed rows will end up in the unrepaired set, in contrast to the rows on the sender site moved to the repaired set. The next repair run will stream the same data back again, causing rows to bounce on and on between nodes on each repair.
See linked dtest on steps to reproduce. An example for reproducing this manually using ccm can be found here