Redo log is a important data to be used to recovery and some other advanced features. A redo entry involved the information about database changing. All of the redo entries should be writen into logfile finally. To avoid disk io waits, oracle allocated a log buffer from sga to cache the redo data. A redo entry is generated in PGA, then it be copied to log buffer by server process. When reach some conditions, LGWR will write the log buffer contents to the logfiles. Log buffer is shared by all of server processes, to protect it, the server process should first request the "redo allocation latch" to allocate buffer from it. Hence, in high concurrent OLTP system, we may observer the redo allocation latch contentions.
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