Question

查询：

UPDATE `cart` SET `user_id` = NULL, `completed` = 0 WHERE `id` = 6948;
Query OK, 0 rows affected (1.21 sec)
Rows matched: 1  Changed: 0  Warnings: 0

影响了0行，但耗时1210毫秒。按id对此行的SELECT总是花费0ms。表大小为（6,354行）。

> show create table cart;
CREATE TABLE `cart` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `user_id` int(11) DEFAULT NULL,
  `completed` tinyint(1) NOT NULL DEFAULT '0' COMMENT 'флаг, указывающий на оформление заказа из данной корзины',
  PRIMARY KEY (`id`),
  KEY `user_id` (`user_id`)
) ENGINE=InnoDB AUTO_INCREMENT=6964 DEFAULT CHARSET=utf8

> show index from cart;
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| cart  |          0 | PRIMARY  |            1 | id          | A         |        6386 |     NULL | NULL   |      | BTREE      |         |               |
| cart  |          1 | user_id  |            1 | user_id     | A         |        2128 |     NULL | NULL   | YES  | BTREE      |         |               |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+

以下是show profile all对此查询的摘录

| Status               | Duration | CPU_user | CPU_system | Context_voluntary | Context_involuntary | Block_ops_in | Block_ops_out | Messages_sent | Messages_received | Page_faults_major | Page_faults_minor | Swaps | Source_function       | Source_file   | Source_line |

| query end            | 2.502555 | 0.003000 |   0.000000 |                88 |                   8 |             0|           136 |             0 |                 0 |                 0 |                 0 |     0 | mysql_execute_command | sql_parse.cc  |        5093 |

为什么block_ops_out是如此不一致，从0到400，需要0ms~2500ms ???如何找到它的根本原因？

服务器版本：10.0.17-MariaDB-1~ wheezy。 VPS根本没有任何明显的负荷。

UPD：在查询后添加了状态变量：

MariaDB> flush status; UPDATE `cart` SET `user_id` = NULL, `completed` = 0 WHERE `id` = 6948; SHOW SESSION STATUS LIKE 'Handler%';
Query OK, 0 rows affected (0.54 sec)

^[[A^[[BQuery OK, 0 rows affected (3.88 sec)
Rows matched: 1  Changed: 0  Warnings: 0

+----------------------------+-------+
| Variable_name              | Value |
+----------------------------+-------+
| Handler_commit             | 2     |
| Handler_delete             | 0     |
| Handler_discover           | 0     |
| Handler_external_lock      | 0     |
| Handler_icp_attempts       | 0     |
| Handler_icp_match          | 0     |
| Handler_mrr_init           | 0     |
| Handler_mrr_key_refills    | 0     |
| Handler_mrr_rowid_refills  | 0     |
| Handler_prepare            | 2     |
| Handler_read_first         | 0     |
| Handler_read_key           | 1     |
| Handler_read_last          | 0     |
| Handler_read_next          | 0     |
| Handler_read_prev          | 0     |
| Handler_read_rnd           | 0     |
| Handler_read_rnd_deleted   | 0     |
| Handler_read_rnd_next      | 0     |
| Handler_rollback           | 0     |
| Handler_savepoint          | 0     |
| Handler_savepoint_rollback | 0     |
| Handler_tmp_update         | 0     |
| Handler_tmp_write          | 0     |
| Handler_update             | 1     |
| Handler_write              | 0     |
+----------------------------+-------+
25 rows in set (0.00 sec)

Answer 1

我所能建议的是尝试将您的表和数据迁移到Mysql MyIsam引擎并执行相同的查询。也许你刚刚发现了MariaDB的弱点，或者有些东西被错误配置或推翻了。

Answer 2

问题不在MySQL / MariaDB中，而是在OpenVZ和服务器上的IO调度中。迁移到更高效的磁盘后，问题就消失了。

MySQL / MariaDB在PRIMARY KEY上缓慢更新

2 个答案: