Oracle Core: Essential Internals for DBAs and Developers

Summary

Oracle uses a write-ahead logging strategy, which means a changed data block cannot be copied to a data file until the description of that change has been written to a log file.

With the exception of a hidden optimization in PL/SQL, and a documented commit option in 10g, transactions are made durable by the rule that the log buffer is written to disk when a transaction commits. The process that does the writing is the log writer (lgwr), and each time a session issues a commit it will post lgwr to write any outstanding log buffer to disk (if lgwr it not already writing), and then go into a wait state until lgwr sends a post to allow it to resume running.

Database blocks are copied to disk in a lazy fashion. There are two key strategies ...