Why are reads slower than writes?
Unlike all major relational databases and some NoSQL systems, Cassandra does not use b-trees and in-place updates on disk. Instead, it uses a sstable/memtable model like Bigtable’s: writes to each ColumnFamily are grouped together in an in-memory structure before being flushed (sorted and written to disk). This means that writes cost no random I/O, compared to a b-tree system which not only has to seek to the data location to overwrite, but also may have to seek to read different levels of the index if it outgrows disk cache!
