had this question posted between the last backup and server breakdown. Meanwhile we switched to Moodle 2.6, but the problem stays the same:
On a course with 2000 participants we have an assignment with about 650 submissions uploaded (each about 200kb). Our teachers want to grade the submissions offline, so they try to export the grading tab. After a long time (over 10 minutes), they receive a blank page. The same issue is for downloading all submitted files as zip.
In my tries, after selecting "download grading tab", there is a significant rise of used cpu% by mysql and apache processes (on a well equipped server - overall load doesn't rise over 5%). There is lots of mem free, no swap used. So I see, the system is working, but seems to hang on some point. When I try to export the grading tab in a smaller course (4 users, 1 submission), it works immediatly.
The only answer to my thread here before the server breakdown was (Thanks Ken!) to enlarge the php.ini's max_exexcution_time to 90 or higher (default value was 30, I now set 90, same for max_input_time, raised memory_limit from 164 to 512). After restarting apache service, the only difference now is a longer time before blank page arises...
Are there any other hints or any experience in downloading grading tabs from such "big" courses?