Permission Denied When i am Trying to take Backup

Hi Gurus,

    Greetings. I am using Postgresql in Red hat linux 71. When i was trying
to take the backup it is giving the error "Permission Denied"
    Did any one faced the Same problem.
    I will tell u what are the things which i did

    1. Login as root
    2. su -l postgres
    3. pg_dump databasename >filename

  Where other things are working fine. Only when i was trying to take the
backup i am getting the problem. Please let me know some guidence

Thanks
Mudhalvan M.M