RocksDB and Pebble until #1470 supported writing a user key across multiple sstables. To maintain LSM invariants, Pebble is forced to compact such sstables together as an 'atomic compaction unit.' Since #1470, Pebble will no longer construct output sstables that share user keys.

We could write a migration that recompacts any sstables within a level that contain a shared user key. A future release would be able to remove the code surrounding atomic compaction units.