Format for parameter LOG_ARCHIVE_FORMAT?

This parameter controls the format of the archive log file name.  This parameter can’t be changed on the fly therefore requires a restart of the instance.  This parameter can be changed with scope=spfile if spfile is used.  

If the format defined in the parameter log_archive_format is invalid the database will startup but archiver will fail to archive logs which will cause database to hang and in the alert log the following message would be reported “ORA-00294: invalid archivelog format specifier..” or you will see an error message when you try to archive the current redo log by running “ALTER SYSTEM ARCHIVE LOG CURRENT;”, so if you change this parameter a quick test can be done by running the above SQL to make sure oracle is able to archive the redo log.

Format options available on 9i:

%s – log sequence number
%S – log sequence number, zero filled
%t – thread number, needed when running RAC as each node creates it’s own archivelog
%T thread number, zero filled, needed when running RAC as each node creates it’s own archivelog

Format options available on 10g

%s – log sequence number
%S – log sequence number, zero filled
%t – thread number, needed when running RAC as each node creates it’s own archivelog
%T – thread number, zero filled, needed when running RAC as each node creates it’s own archivelog
%a – activation ID
%d – database ID
%r – resetlogs ID that ensures unique names are constructed for the archived log files across multiple incarnations of the database.

In 10g, %s, %t, %r are required to be present in the parameter, if it doesn’t the database fail to start with the error ORA-19905: log_archive_format must contain %s, %t and %r.  Using this format makes it the archive log filename unique for that instance.

The following article has information how to start database in archivelog mode