1.2 makes LCS run parallel, though if L0 compaction get far behind and concurrent compactions at L0 where each compaction holds maximum number of SSTable to compact at L0(32), it will likely cause overlap in L1. There will be ERROR log as follows:
ERROR [CompactionExecutor:30] 2013-08-19 17:54:29,648 LeveledManifest.java (line 244) At level 1, SSTableReader(path='xxx-Data.db') [DecoratedKey(204853724659241194183955214890519, 30303132343830), DecoratedKey(69227335985728660912035125310473966323, 30393537373332)] overlaps SSTableReader(path='xxx-Data.db') [DecoratedKey(217896711032704014921095870827202, 30333635363932), DecoratedKey(71430242198281555888954138354238066233, 30333035343132)]. This is caused by a bug in Cassandra 1.1.0 .. 1.1.3. Sending back to L0. If you have not yet run scrub, you should do so since you may also have rows out-of-order within an sstable
We should send back compacted SSTables to L0 when compacting max SSTables at L0. Also, the above error message is confusing, at version 1.2, we can reduce to WARNing level without mentioning scrub.
C* 2.0 performs Size-Tiered compaction on L0 when it has max SSTables and sends back compacted SSTable to L0, so I think we don't need to fix this on 2.0.