A High Level Comparison Between Oracle and SQL Server – Part IV
Log vs. Alert Log
SQL Server maintains a running log of its
operations. This log includes information about the start-up and recovery
events, user actions, backups, configuration changes, invalid login attempts,
errors, warnings etc. Every time the SQL service starts up, a new log file is
created. This log file is known as the SQL Server Error Log.
The error log is a prime source of
information for DBAs troubleshooting incidents. By default SQL Server keeps the
last six log files in a rolling fashion with the oldest one being deleted
first. This default behaviour can be changed and SQL can be configured to keep
a pre-defined number of files. The current error log file is named ERRORLOG (without
any extension) while the one before it is named ERRORLOG.1, the one before that
is ERRORLOG.2 and so on.
Oracle’s equivalent of SQL Server Error Log
is the Alert Log file. Alert Log contains information about database
start-up and shutdowns, instance recovery, configuration changes, internal
database errors, initialisation parameter values etc.
Unlike the SQL error log, the Oracle alert log
does not create a new file every time the instance is restarted. In fact Oracle
maintains only one Alert Log file which can grow quite big as information is
accumulated over time. An Alert Log will always have the name of alert_<instance>.log
where <instance> the Oracle instance name. Like SQL Server DBAs,
Oracle DBAs also use the Alert Log for troubleshooting purposes.
Both SQL Server error log and the Oracle alert
log are plain ASCII text files that can be opened in any text editor. SQL
Server Management Studio allows viewing the error log from a Windows interface
and Oracle’s Enterprise Manager Database Control also enables viewing the alert
log from a web based interface.
The location of the SQL Server error log is
dictated by a registry entry. The default location is usually under the LOG
directory in the SQL Server installation folder. For Oracle, the location of
the alert log is determined by an initialisation parameter called BACKGROUND_DUMP_DEST
and it is usually the bdump folder under the $ORACLE_HOME directory.