InnoDB read ahead still active with random_read_ahead OFF

[jocelyn fournier]

Hi,

In Percona 5.7, it seems even with innodb_random_read_ahead=OFF, innodb is still doing a few read ahead. This is a behavior I don't observe on 5.6.

mysql> show global variables LIKE '%innodb_version%';
+----------------+-----------+
| Variable_name | Value |
+----------------+-----------+
| innodb_version | 5.7.18-14 |
+----------------+-----------+
1 row in set (0.00 sec)

mysql> show global variables LIKE '%ahead%';
+-----------------------------+-------+
| Variable_name | Value |
+-----------------------------+-------+
| innodb_log_write_ahead_size | 512 |
| innodb_random_read_ahead | OFF |
| innodb_read_ahead_threshold | 56 |
+-----------------------------+-------+
3 rows in set (0.00 sec)

mysql> show global status LIKE '%ahead%';
+---------------------------------------+---------+
| Variable_name | Value |
+---------------------------------------+---------+
| Innodb_buffer_pool_read_ahead_rnd | 0 |
| Innodb_buffer_pool_read_ahead | 1177940 |
| Innodb_buffer_pool_read_ahead_evicted | 0 |
+---------------------------------------+---------+
3 rows in set (0.00 sec)

mysql> show global status LIKE '%ahead%';
+---------------------------------------+---------+
| Variable_name | Value |
+---------------------------------------+---------+
| Innodb_buffer_pool_read_ahead_rnd | 0 |
| Innodb_buffer_pool_read_ahead | 1177983 |
| Innodb_buffer_pool_read_ahead_evicted | 0 |
+---------------------------------------+---------+

Is this expected?

Thanks and regards,
  Jocelyn Fournier

[Shahriyar Rzayev (rzayev-sehriyar)]

Percona now uses JIRA for bug reports so this bug report is migrated to: https://jira.percona.com/browse/PS-3707