Why log file is rewritten at each session execution? Is it possible to have a separate log for each session run?

By default, each time the session is executed, the log file is overwritten and the old information is deleted.  If you need to have a separate log for each session execution you just need to check “Create separate log file for each execution” checkbox in the “Settings” window.

Click "Settings" button at the bottom of the DBConvert window and in the appeared window you will see this checkbox.

 You can get the "Settings" at any stage you are.