Controls the durability/speed trade-off for commits. Set to 0 (write and flush redo log to disk only once per second), 1 (flush to disk at each commit), 2 (write to log at commit but flush to disk only once per second) or 3 (flush to disk at prepare and at commit, slower and usually redundant). 1 and 3 guarantees that after a crash, committed transactions will not be lost and will be consistent with the binlog and other transactional engines. 2 can get inconsistent and lose transactions if there is a power failure or kernel crash but not if mysqld crashes. 0 has no guarantees in case of crash. 0 and 2 can be faster than 1 or 3.