We need to keep sharding metadata consistent with tablet mapping to
shards in order for node restart to detect that those sstables belong
to a single shard and that resharding is not necessary. Resharding of
sstables based on tablet metadata is not implemented yet and will
abort after this series.
Keeping sharding metadata accurate for tablets is only necessary until
compaction group integration is finished. After that, we can use the
sstable token range to determine the owning tablet and thus the owning
shard. Before that, we can't, because a single sstable may contain
keys from different tablets, and the whole key range may overlap with
keys which belong to other shards.
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