diff --git a/admin/tool/log/store/legacy/classes/task/cleanup_task.php b/admin/tool/log/store/legacy/classes/task/cleanup_task.php
index 474604b4a74..4d0e62aa926 100644
--- a/admin/tool/log/store/legacy/classes/task/cleanup_task.php
+++ b/admin/tool/log/store/legacy/classes/task/cleanup_task.php
@@ -44,11 +44,26 @@ class cleanup_task extends \core\task\scheduled_task {
     public function execute() {
         global $CFG, $DB;
 
-        // Delete old logs to save space (this might need a timer to slow it down...).
-        if (!empty($CFG->loglifetime)) { // Value in days.
-            $loglifetime = time(0) - ($CFG->loglifetime * 3600 * 24);
-            $DB->delete_records_select("log", "time < ?", array($loglifetime));
-            mtrace(" Deleted old legacy log records");
+        if (empty($CFG->loglifetime)) {
+            return;
         }
+
+        $loglifetime = time() - ($CFG->loglifetime * 3600 * 24); // Value in days.
+        $lifetimep = array($loglifetime);
+        $start = time();
+
+        while ($min = $DB->get_field_select("log", "MIN(time)", "time < ?", $lifetimep)) {
+            // Break this down into chunks to avoid transaction for too long and generally thrashing database.
+            // Experiments suggest deleting one day takes up to a few seconds; probably a reasonable chunk size usually.
+            // If the cleanup has just been enabled, it might take e.g a month to clean the years of logs.
+            $params = array(min($min + 3600 * 24, $loglifetime));
+            $DB->delete_records_select("log", "time < ?", $params);
+            if (time() > $start + 300) {
+                // Do not churn on log deletion for too long each run.
+                break;
+            }
+        }
+
+        mtrace(" Deleted old legacy log records");
     }
 }
diff --git a/admin/tool/log/store/legacy/tests/store_test.php b/admin/tool/log/store/legacy/tests/store_test.php
index 7fb6353b64d..c77354496fd 100644
--- a/admin/tool/log/store/legacy/tests/store_test.php
+++ b/admin/tool/log/store/legacy/tests/store_test.php
@@ -273,4 +273,39 @@ class logstore_legacy_store_testcase extends advanced_testcase {
             $this->assertContains($expectedreport, $reports);
         }
     }
+
+    /**
+     * Test that the legacy log cleanup works correctly.
+     */
+    public function test_cleanup_task() {
+        global $DB;
+
+        $this->resetAfterTest();
+
+        // Create some records spread over various days; test multiple iterations in cleanup.
+        $record = (object) array('time' => time());
+        $DB->insert_record('log', $record);
+        $record->time -= 3600 * 24 * 30;
+        $DB->insert_record('log', $record);
+        $record->time -= 3600 * 24 * 30;
+        $DB->insert_record('log', $record);
+        $record->time -= 3600 * 24 * 30;
+        $DB->insert_record('log', $record);
+        $this->assertEquals(4, $DB->count_records('log'));
+
+        // Remove all logs before "today".
+        set_config('loglifetime', 1);
+
+        try {
+            ob_start();
+            $clean = new \logstore_legacy\task\cleanup_task();
+            $clean->execute();
+            ob_end_clean();
+        } catch (Exception $e) {
+            ob_end_clean();
+            throw $e;
+        }
+
+        $this->assertEquals(1, $DB->count_records('log'));
+    }
 }
diff --git a/admin/tool/log/store/standard/classes/task/cleanup_task.php b/admin/tool/log/store/standard/classes/task/cleanup_task.php
index cc27b1fa091..2d6842318cc 100644
--- a/admin/tool/log/store/standard/classes/task/cleanup_task.php
+++ b/admin/tool/log/store/standard/classes/task/cleanup_task.php
@@ -43,12 +43,29 @@ class cleanup_task extends \core\task\scheduled_task {
      */
     public function execute() {
         global $DB;
+
         $loglifetime = (int)get_config('logstore_standard', 'loglifetime');
 
-        if ($loglifetime > 0) {
-            $loglifetime = time() - ($loglifetime * 3600 * 24); // Value in days.
-            $DB->delete_records_select("logstore_standard_log", "timecreated < ?", array($loglifetime));
-            mtrace(" Deleted old log records from standard store.");
+        if (empty($loglifetime) || $loglifetime < 0) {
+            return;
         }
+
+        $loglifetime = time() - ($loglifetime * 3600 * 24); // Value in days.
+        $lifetimep = array($loglifetime);
+        $start = time();
+
+        while ($min = $DB->get_field_select("logstore_standard_log", "MIN(timecreated)", "timecreated < ?", $lifetimep)) {
+            // Break this down into chunks to avoid transaction for too long and generally thrashing database.
+            // Experiments suggest deleting one day takes up to a few seconds; probably a reasonable chunk size usually.
+            // If the cleanup has just been enabled, it might take e.g a month to clean the years of logs.
+            $params = array(min($min + 3600 * 24, $loglifetime));
+            $DB->delete_records_select("logstore_standard_log", "timecreated < ?", $params);
+            if (time() > $start + 300) {
+                // Do not churn on log deletion for too long each run.
+                break;
+            }
+        }
+
+        mtrace(" Deleted old log records from standard store.");
     }
 }
diff --git a/admin/tool/log/store/standard/tests/store_test.php b/admin/tool/log/store/standard/tests/store_test.php
index 730714dd774..23315565dbb 100644
--- a/admin/tool/log/store/standard/tests/store_test.php
+++ b/admin/tool/log/store/standard/tests/store_test.php
@@ -276,4 +276,46 @@ class logstore_standard_store_testcase extends advanced_testcase {
         get_log_manager(true);
     }
 
+    /**
+     * Test that the standard log cleanup works correctly.
+     */
+    public function test_cleanup_task() {
+        global $DB;
+
+        $this->resetAfterTest();
+
+        // Create some records spread over various days; test multiple iterations in cleanup.
+        $ctx = context_course::instance(1);
+        $record = (object) array(
+            'edulevel' => 0,
+            'contextid' => $ctx->id,
+            'contextlevel' => $ctx->contextlevel,
+            'contextinstanceid' => $ctx->instanceid,
+            'userid' => 1,
+            'timecreated' => time(),
+        );
+        $DB->insert_record('logstore_standard_log', $record);
+        $record->timecreated -= 3600 * 24 * 30;
+        $DB->insert_record('logstore_standard_log', $record);
+        $record->timecreated -= 3600 * 24 * 30;
+        $DB->insert_record('logstore_standard_log', $record);
+        $record->timecreated -= 3600 * 24 * 30;
+        $DB->insert_record('logstore_standard_log', $record);
+        $this->assertEquals(4, $DB->count_records('logstore_standard_log'));
+
+        // Remove all logs before "today".
+        set_config('loglifetime', 1, 'logstore_standard');
+
+        try {
+            ob_start();
+            $clean = new \logstore_standard\task\cleanup_task();
+            $clean->execute();
+            ob_end_clean();
+        } catch (Exception $e) {
+            ob_end_clean();
+            throw $e;
+        }
+
+        $this->assertEquals(1, $DB->count_records('logstore_standard_log'));
+    }
 }