Sstable, which stands for Sorted String Table, is a data structure used for storing key-value pairs. An sstable is essentially a sorted table of key-value pairs, where each key is associated with a corresponding value.Each level is 10x the size of the previous one and each sstable is 160MB by default. L0 is where sstables are streamed/flushed - no overlap guarantees are given here. When picking compaction candidates we have to make sure that the compaction does not create overlap in the target level.
