Just as you enabled Document log...

Select Montiors toolbar option > Configure monitors > Session backups (tab) > File (sub-tab)

HI Mark,

Thanks for you answer, I want to know further can I create some custom log file to write records which got failed due to error with record Id and error description into it.

If yes please let me know some sample script to write that into file. along with the process followed. 

I have not done extensive logging, however, I do know there are "pif" Info/Warning/Error functions that can be utilized (in a mapping) to capture any values / messages.  Maybe this is enough to populate the log file you enabled under "Config monitors".  There is an "Additional" sub-tab (in a mapping when root node highlighted) where global variables can be declared along with a default On Open/ On Close scripting section, so maybe there you can open a new logging file, which gets populated during mapping processing (via pif functions), then close logging file when processing is done.  You will help yourself immensely by reviewing Help for all of this (ie. logging, file management, pif functions).  Sorry I cannot be of more help.