Both MeteredFlusher and BatchlogManager share the same OptionalTasks thread. So, when batchlog manager processes its tasks no flushes can occur. Even more, batchlog manager waits for batchlog CF compaction to finish.
On a lot of batchlog activity this prevents memtables from flush for a long time, making the node OOM.
Fixed this by moving batchlog to its own thread and not waiting for batchlog compaction to finish.