http://www.tuicool.com/articles/NzAFZn
https://github.com/percona/percona-server/pull/83/commits/0910ae6f52d0e7725a94cb5236115d17f0220c1a
show engine innodb status
pt-deadlock-logger
innodb_print_all_deadlocks={on|off}
mysql> show variables like "%purge%"; +-----------------------------------------+-------+ | Variable_name | Value | +-----------------------------------------+-------+ | gtid_purged | | | innodb_max_purge_lag | 0 | | innodb_max_purge_lag_delay | 0 | | innodb_purge_batch_size | 300 | | innodb_purge_run_now | OFF | | innodb_purge_stop_now | OFF | | innodb_purge_threads | 1 | | innodb_trx_purge_view_update_only_debug | OFF | | relay_log_purge | ON | +-----------------------------------------+-------+ 9 rows in set (0.02 sec)
set global innodb_purge_stop_now=1
会话1:
mysql> CREATE TABLE t1 ( -> a INT NOT NULL, -> b INT NOT NULL, -> PRIMARY KEY(b), -> UNIQUE KEY(a)) ENGINE=INNODB; Query OK, 0 rows affected (0.23 sec) mysql> INSERT INTO t1 VALUES (1,1),(2,2); Query OK, 2 rows affected (0.16 sec) Records: 2 Duplicates: 0 Warnings: 0 mysql> DELETE FROM t1; Query OK, 2 rows affected (0.17 sec) mysql> begin; Query OK, 0 rows affected (0.00 sec) mysql> REPLACE INTO t1 VALUES (1,2); Query OK, 1 row affected (0.00 sec)
会话2：
show engine innodb statusG
---TRANSACTION 107849, ACTIVE 4 sec 4 lock struct(s), heap size 1248, 4 row lock(s), undo log entries 1 MySQL thread id 3, OS thread handle 0x2ab31a1d2940, query id 49 localhost root cleaning up TABLE LOCK table `test`.`t1` trx id 107849 lock mode IX RECORD LOCKS space id 237 page no 3 n bits 72 index `PRIMARY` of table `test`.`t1` trx id 107849 lock_mode X locks rec but not gap Record lock, heap no 3 PHYSICAL RECORD: n_fields 4; compact format; info bits 0 0: len 4; hex 80000002; asc ;; 主健锁主的记录锁 1: len 6; hex 00000001a549; asc I;; 2: len 7; hex 2c000001b0188e; asc , ;; 3: len 4; hex 80000001; asc ;;
RECORD LOCKS space id 237 page no 4 n bits 72 index `a` of table `test`.`t1` trx id 107849 lock_mode X //已经删除了的记录 [1,1] heap no 2 Record lock, heap no 2 PHYSICAL RECORD: n_fields 2; compact format; info bits 32 0: len 4; hex 80000001; asc ;; 1: len 4; hex 80000001; asc ;;
Record lock, heap no 3 PHYSICAL RECORD: n_fields 2; compact format; info bits 32 //已经删除了的记录 [2,2] heap no 3 0: len 4; hex 80000002; asc ;; 1: len 4; hex 80000002; asc ;;
RECORD LOCKS space id 237 page no 4 n bits 72 index `a` of table `test`.`t1` trx id 107849 lock_mode X locks gap before rec Record lock, heap no 4 PHYSICAL RECORD: n_fields 2; compact format; info bits 0 //heap no 4 0: len 4; hex 80000001; asc ;; 二级索引CAP锁 1: len 4; hex 80000002; asc ;;
[MySQL bug] unique key corruption again….. 原文 http://mysqllover.com/?p=1477 最近Percona的研发人员report了一个uk corruption的bug，这个Bug不同于之前发现的bug（见我的另外一篇博客http://mysqllover.com/?p=1041），而是影响从5.1 到5.8全系列MySQL版本，应该算是设计上的缺陷吧。 创建测试表 CREATE TABLE t1 ( a INT NOT NULL, b INT NOT NULL, PRIMARY KEY(b), UNIQUE KEY(a)) ENGINE=INNODB; INSERT INTO t1 VALUES (1,1),(2,2);
0. 停止Purge操作：SET GLOBAL innodb_purge_stop_now = ON; 防止标记删除的记录被purge掉 删除表上的数据：DELETE FROM t1; //这时候表上物理记录还存在，只是被标记删除了。 1. SESSION 1: REPLACE INTO t1 VALUES (1,2); 由于有一条标记删除的记录，检查pk上duplicate key，在聚集索引上加锁：mode=1027 (1024 +3 = LOCK_REC_NOT_GAP | LOCK_X) 堆栈： row_ins_clust_index_entry |--> row_ins_clust_index_entry_low |--> row_ins_duplicate_error_in_clust |--> row_ins_set_exclusive_rec_lock //x record lock |-->lock_clust_rec_read_check_and_lock
检查二级索引duplicate key，由于存在标记删除的记录，这里需要加锁，类型为X锁 堆栈： row_ins_index_entry |-->row_ins_sec_index_entry |--> row_ins_sec_index_entry_low |--> row_ins_scan_sec_index_for_duplicate |--> row_ins_set_exclusive_rec_lock //x record lock |--> lock_sec_rec_read_check_and_lock
当完成duplicate key检测后 （当然这里是成功的），我们让SESSION1稍微等一会（设置DEBUG SYNC同步点）
2. SESSION 2：REPLACE INTO t1 VALUES (1,3); 尝试插入第二条记录，这条记录和SESSION1插入的uk是冲突的，pk不冲突，因此先插入pk成功，然后检查uk上的duplicate key加锁：mode =3，但是session1已经在uk为1的记录上加X锁了，因此Session 2进入锁等待 加锁堆栈： row_ins_index_entry |--> row_ins_sec_index_entry |--> row_ins_sec_index_entry_low |--> row_ins_scan_sec_index_for_duplicate |--> row_ins_set_exclusive_rec_lock |--> lock_sec_rec_read_check_and_lock
mysql> select * from information_schema.innodb_locks; +------------+-------------+-----------+-----------+-------------+------------+------------+-----------+----------+-----------+ | lock_id | lock_trx_id | lock_mode | lock_type | lock_table | lock_index | lock_space | lock_page | lock_rec | lock_data | +------------+-------------+-----------+-----------+-------------+------------+------------+-----------+----------+-----------+ | 1300:6:4:2 | 1300 | X | RECORD | `test`.`t1` | a | 6 | 4 | 2 | 1 | | 1299:6:4:2 | 1299 | X | RECORD | `test`.`t1` | a | 6 | 4 | 2 | 1 | +------------+-------------+-----------+-----------+-------------+------------+------------+-----------+----------+-----------+ 2 rows in set (0.00 sec)
3. 开启Purge操作：SET GLOBAL innodb_purge_run_now=ON; 后台Purge线程会去清理标记删除的二级索引记录，但之前session1和session2都有请求uk上的排他锁，记录没有了，这些锁对象也需要做对应的处理，理论上锁请求应该被下一条记录锁继承，
并转换成GAP锁 堆栈： row_purge_step |--> row_purge |--> row_purge_record_func |--> row_purge_del_mark |--> row_purge_remove_sec_if_poss |--> row_purge_remove_sec_if_poss_leaf |--> btr_cur_optimistic_delete_func |--> lock_update_delete |--> lock_rec_inherit_to_gap
我们来看看锁继承的逻辑，函数为lock_rec_inherit_to_gap： for (lock = lock_rec_get_first(block, heap_no);lock != NULL;lock = lock_rec_get_next(heap_no, lock) )
{
if(!lock_rec_get_insert_intention(lock)&&
!( ( srv_locks_unsafe_for_binlog || lock->trx->isolation_level<=TRX_ISO_READ_COMMITTED) && lock_get_mode(lock) == LOCK_X ) )
{
lock_rec_add_to_queue(LOCK_REC | LOCK_GAP | lock_get_mode(lock),heir_block, heir_heap_no, lock->index,lock->trx, FALSE); } }
说明: 对于如下场景，不会做锁继承： a. 锁类型为插入意向锁 b. srv_locks_unsafe_for_binlog打开且锁类型为X锁 c. 锁对应事务的隔离级别小于等于RC且锁类型为X锁 由于在执行类似REPLACE， LOAD DATAFILE REPLACE， INSERT ON DUPLICATE KEY UPDATE等操作时，当检查duplicate key时加的是X锁，本测试样例的隔离级别为RC，因此锁对象未被继承，而是
直接解除了，随后等待锁的线程(session2)被唤醒（lock_update_delete –> lock_rec_reset_and_release_wait）。 从IS表也可以看出来这一变化： mysql> select * from information_schema.innodb_locks; Empty set (0.00 sec) 此时innodb_locks里面已经没有记录了，purge操作“悄悄”的破坏了InnoDB的锁协议。 随后唤醒SESSION2继续操作。 4. SESSION 2: 由于获得了记录锁，因此可以继续插入记录 5. SESSION 1：由于已经完成了duplicate key检查，因此可以继续插入记录 mysql> select * from t1; +---+---+ | a | b | +---+---+ | 1 | 2 | | 1 | 3 | +---+---+ 2 rows in set (0.00 sec) mysql> check table t1; +---------+-------+----------+-----------------------------------------------+ | Table | Op | Msg_type | Msg_text | +---------+-------+----------+-----------------------------------------------+ | test.t1 | check | Warning | InnoDB: The B-tree of index "a" is corrupted. | | test.t1 | check | error | Corrupt | +---------+-------+----------+-----------------------------------------------+ 2 rows in set (0.00 sec) 问题及解决 问题的原因Alexey其实在bug上已经解释的很清楚，Innodb认为只可能加S锁来维持一致性约束，因此当记录被物理删除时，只有S类型的锁才被继承。但对于REPLACE这样的操作，加的是X类型的锁，这种锁类型必须也要考虑进去，将其继承给下一条记录。Alexey已经将patch push到percona server，改动也就一行： Ref: bug链接 https://bugs.mysql.com/bug.php?id=76927 补丁地址： https://github.com/percona/percona-server/pull/83