Hi Mark,
Thanks for your relply and help.
I do have /home/venny/moodledata/temp/ in my Moodle, and after I run backup, I can see such a directory /home/venny/moodledata/temp/backup/1181833080/
The problem is that subdirectories under /home/venny/moodledata/temp/backup/1181833080/ is 644.
That's why the system can't write down the backup file under the subdirectories?
By the way, what permissions should I set to the moodle or moodledata directories instead of 777?
Thanks again for your help.