Explanation http://docs.oracle.com/cd/B28359_01/server.111/b28310/diag001.htm#ADMIN11008 The Automatic DiagnosticRepository (ADR) is a directory structure that is stored outside of the database. It is therefore available for problem diagnosis when the database is down. The ADR root directory is known as ADR base. Its location is set by the DIAGNOSTIC_DEST initialization parameter. If this parameter is omitted or left null, the database sets DIAGNOSTIC_DEST upon startup as follows: If environment variable ORACLE_BASE is set, DIAGNOSTIC_DEST is set to the directory designated by ORACLE_BASE. If environment variable ORACLE_BASE is not set, DIAGNOSTIC_DEST is set to ORACLE_HOME/log.