gandolf,
Thank you for your answer, I already read the Oracle documents (Oracle Database Concepts) and I didn't find the answers.
I don't find the process that writes the rollback data in the data files and i don't know when these data blocks are written; indeed, in documents, they always say that DBW writes DIRTY data block to data files... normally, when the DBW writes the "after" image of the non committed data in the table segment, it should write the "before" image in the rollback segment... but i am not sure of what i'm saying !!!

davey23uk,
thank you for your precision.