When you create an application in Cognos Planning contributor, the log file of this SQL database is mdf instead of ldf.

You can change this by editing the epEAdminSQL7Resources.xml located in the <installation directory>\c8\bin

search for string (line 6) 

LOG ON  ( NAME = [%1_log],  FILENAME = N'%2%1_log.mdf',  SIZE = 10MB,  FILEGROWTH = 10% )

Change mdf to ldf

LOG ON  ( NAME = [%1_log],  FILENAME = N'%2%1_log.ldf',  SIZE = 10MB,  FILEGROWTH = 10% )

save file and restart Cognos services to let the change take effect

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