Generalized binary search tree. Each node can have more than 1 child. Keep the number of nodes at each leaf between d and 2d. Split/combine when out of this range.
B+ trees generally have pointers connecting leaves to speed sequential scans.
In B+ tree, only leaf nodes store data; internal nodes store copies of keys. In B trees, internal nodes store data.
http://www.benstopford.com/2015/02/14/log-structured-merge-trees/
In-memory table for fast writes. Periodically flush to disk, creating SSTables (sorted string tables) on disk. Index for each SSTable stored in memory. SSTables periodically merged. SSTables are sorted and immutable.
Reads check in-memory table and SSTable indexes, if present in SSTable, fetch from SSTable. Can use in-memory index and bloom filters to speed up reads.