On our production site our staff programmer Cliff found this issue, when we added the winter/spring 2008 students to the DB, then course level manual backup will not complete. We did some debugging and found that the function user_check_backup in /backup/backuplib.php were not implemented efficiently and would generate big memory footprint:

function user_check_backup($course,$backup_unique_code,$backup_users,$backup_messages) {
//$backup_users=0-->all
// 1-->course (needed + enrolled)
// 2-->none

global $CFG;
global $db;

$count_users = 0;

//If we've selected none, simply return 0
if ($backup_users == 0 or $backup_users == 1) {

//Calculate needed users (calling every xxxx_get_participants function + scales users)
$needed_users = backup_get_needed_users($course, $backup_messages);

//Calculate enrolled users (students + teachers)
$enrolled_users = backup_get_enrolled_users($course);

//Calculate all users (every record in users table)
$all_users = backup_get_all_users();

//Calculate course users (needed + enrolled)
//First, needed
$course_users = $needed_users;

//Now, enrolled
if ($enrolled_users) {
foreach ($enrolled_users as $enrolled_user)

{ $course_users[$enrolled_user->id]->id = $enrolled_user->id; }

}

//Now, depending of parameters, create $backupable_users
if ($backup_users == 0)

{ $backupable_users = $all_users; }

else

{ $backupable_users = $course_users; }

We rewrote it a little bit and are still testing the patch (patch is not included here in this ticket right now), but would like to create a bug ticket here so that people who are running large sites would aware of this issue. Thanks!