The "NoSQL" Discussion has Nothing to Do With SQL | blog@CACM | Communications of the ACM

http://cacm.acm.org/blogs/blog-cacm/50678-the-nosql-discussion-has-nothing-to-do-with-sql/fulltext
Using either stored procedures or embedding, the useful work component is a very small percentage of total transaction cost, for today’s OLTP data bases which usually fit in main memory. Instead, a recent paper [1] calculated that total OLTP time was divided almost equally between the following four overhead components:
Logging: Traditional databases write everything twice; once to the database and once to the log. Moreover, the log must be forced to disk, to guarantee transaction durability. Logging is, therefore, an expensive operation.
Locking: Before touching a record, a transaction must set a lock on it in the lock table. This is an overhead-intensive operation.
Latching: Updates to shared data structures (B-trees, the lock table, resource tables, etc.) must be done carefully in a multi-threaded environment. Typically, this is done with short-term duration latches, which are another considerable source of overhead.
Buffer Management: Data in traditional systems is stored on fixed-size disk pages. A buffer pool manages which set of disk pages is cached in memory at any given time. Moreover, records must be located on pages and the field boundaries identified. Again, these operations are overhead intensive.