Moodle
  1. Moodle
  2. MDL-20018

CLONE - Scheduled course backup runs full table scan on mdl_backup_log - possible regression.

    Details

    • Type: Improvement Improvement
    • Status: Closed
    • Priority: Minor Minor
    • Resolution: Won't Fix
    • Affects Version/s: 1.8.4, 1.9
    • Fix Version/s: None
    • Component/s: Backup, Performance
    • Labels:
      None
    • Environment:
      MySQL 5.0.22
    • Database:
      MySQL
    • Affected Branches:
      MOODLE_18_STABLE, MOODLE_19_STABLE

      Description

      If scheduled course backups are enabled, cron.php calls schedule_backup_cron() whenever it runs. Line 36 of backup/backup_scheduled.php is

      $numofrec = count_records_select ("backup_log","time > $timeafter");

      This runs a full scan on mdl_backup_log, which can be a very large table depending on your site's log-retention policy. For example, we have cron.php scheduled to run every 5 minutes, and ~65,000 rows are read, simply to find out whether a backup task is currently running. The query takes around 4 seconds to complete. A workaround is to add an index to the table.

      ALTER TABLE mdl_backup_log ADD INDEX (time);

      This avoids the full table scan, and the query now completes in less than one hundredth of a second at our site. The drawback is that mdl_backup_log.MYI is now almost as big as mdl_backup_log.MYD I can't see any way of working around this issue in the PHP code.

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                • Created:
                  Updated:
                  Resolved: