Tlog just record DML ?

Dear:

   I met " 

Msg 3023, Level 16, 

Backup, file manipulation operations (such as ALTER DATABASE ADD FILE) and encryption changes on a database must be serialized. Reissue the statement after the current backup or file manipulation operation is completed.

"

Does it means I can not add new datafile during the database backup is running ?  Since sql server has TLOG , does not

it remember the datafiles changed, right ?  It was too weak.