When selecting L0 files in the stripe store file manager to compact, they all will be selected. This is the key problem. No file count control and no compaction size control for L0 files compactions now. If the compaction size is large, e.g. some TBs, then the L0 compaction will need a lot of time to complete.
Since L0 files not only contains the recently flushed files, bulk loaded files will also be put into L0. And what's more, when opening a daughter region, if the parent stripes can not be rebuild in the daughter, all the files will be put to L0.
So when there are large enough files in L0, there will exists a quite long compaction for all the L0 files. If the compaction speed less than the file flush speed to L0, larger compactions afterwards. This is a big problem especially in bulkloading files.