The unsung heroes of InnoDB are the logfiles. They are what makes InnoDB automatic crash recovery possible.
Database administrators of other DBMS may be familiar with the concept of a “redo” log. When data is changed, affected data pages are changed in the innodb_buffer_pool. Then, the change is written to the redo log, which in MySQL is the InnoDB logfile (ib_logfile0 and ib_logfile1). The pages are marked as “dirty”, and eventually get flushed and written to disk.
If MySQL crashes, there may be data that is changed that has not been written to disk. Those data pages were marked as “dirty” in the innodb_buffer_pool, but after a MySQL crash the innodb_buffer_pool no longer exists. However, they were written to the redo log. On crash recovery, MySQL can read the redo log (InnoDB log files) and apply any changes that were not written to[Read more...]