You can set the path for sql_server.log using this keyword in ctsrvr.cfg:
SETENV TPE_LOG=<directory>
; SQL statement diagnostic logging
;SQL_DEBUG LOG_STMT
;SQL_SERVER_LOG_SIZE 100MB
;SETENV TPE_LOG=<alternate path to sql_server.log>
You may find this section present and commented out in your default FairCom DB configuration file for your convenience.
See Also