Another key difference between database systems is how they handle updates to the physical records stored on disk.
Relational databases, such as MySQL, maintain a variety of structures in both the memory and disk, where writes from in-flight transactions and writes from completed transactions are persisted. Once the transaction has been committed, the physical record on disk for a given key is updated to reflect that. On the other hand, many NoSQL databases, such as HBase and Cassandra, are variants of what is called a log-structured merge (LSM) database.
In such an LSM database, updates aren't applied to the record at transaction commit. Instead, updates are applied in memory. Once the memory structure gets ...
