MySQL数据库会莫名其妙地重新启动(adaptive hash index)

2024年 2月 19日 29.5k 0

一次排除因为自适应哈希索引(adaptive hash index)造成的MySQL数据库崩溃的经历,并探讨MySQL在何种情况下应该激活自适应哈希索引。

01

MySQL数据库崩溃

一个客户的MySQL数据库隔一段时间就会莫名其妙地重新启动,在错误日志对应的时间点里下面的记录:

    --Thread 140508672222976 has waited at row0purge.cc line 113 for 241 seconds the semaphore:
    X-lock on RW-latch at 0x7fcac68e7b80 created in file buf0buf.cc line 1474
    a writer (thread id 140508724832000) has reserved it in mode SX
    number of readers 0, waiters flag 1, lock_word: 10000000
    Last time read locked in file btr0sea.cc line 1037
    Last time write locked in file build/mysql-5.7-pMP6e2/mysql-5.7-5.7.42/storage/innobase/buf/buf0flu.cc line 1213
    InnoDB: ###### Starts InnoDB Monitor for 30 secs to print diagnostic info:
    InnoDB: Pending preads 0, pwrites 0


    =====================================
    2024-01-31 18:09:35 0x7fcaba7f0700 INNODB MONITOR OUTPUT
    =====================================
    Per second averages calculated from the last 2 seconds
    -----------------
    BACKGROUND THREAD
    -----------------
    srv_master_thread loops: 118905 srv_active, 0 srv_shutdown, 878621 srv_idle
    srv_master_thread log flush and writes: 997526
    ----------
    SEMAPHORES
    ----------
    OS WAIT ARRAY INFO: reservation count 1346616
    --Thread 140502119106304 has waited at row0upd.cc line 2882 for 236 seconds the semaphore:
    X-lock on RW-latch at 0x7fcac6937708 created in file buf0buf.cc line 1474
    a writer (thread id 140508724832000) has reserved it in mode SX
    number of readers 0, waiters flag 1, lock_word: 10000000
    Last time read locked in file btr0sea.cc line 1037
    Last time write locked in file build/mysql-5.7-pMP6e2/mysql-5.7-5.7.42/storage/innobase/buf/buf0flu.cc line 1213
    --Thread 140508680615680 has waited at log0log.cc line 1746 for 198 seconds the semaphore:
    S-lock on RW-latch at 0x579b7e0 created in file log0log.cc line 846
    a writer (thread id 140508680615680) has reserved it in mode exclusive
    number of readers 0, waiters flag 1, lock_word: 0
    Last time read locked in file log0log.cc line 1746
    Last time write locked in file build/mysql-5.7-pMP6e2/mysql-5.7-5.7.42/storage/innobase/log/log0log.cc line 1638
    --Thread 140508672222976 has waited at row0purge.cc line 113 for 242 seconds the semaphore:
    X-lock on RW-latch at 0x7fcac68e7b80 created in file buf0buf.cc line 1474
    a writer (thread id 140508724832000) has reserved it in mode SX
    number of readers 0, waiters flag 1, lock_word: 10000000
    Last time read locked in file btr0sea.cc line 1037
    Last time write locked in file build/mysql-5.7-pMP6e2/mysql-5.7-5.7.42/storage/innobase/buf/buf0flu.cc line 1213
    OS WAIT ARRAY INFO: signal count 1471450
    RW-shared spins 0, rounds 971756, OS waits 214261
    RW-excl spins 0, rounds 7944761, OS waits 111771
    RW-sx spins 68624, rounds 1162189, OS waits 17279
    Spin rounds per wait: 971756.00 RW-shared, 7944761.00 RW-excl, 16.94 RW-sx
    ------------
    TRANSACTIONS
    ------------

    从出错信息看,是锁竞争造成线程等待引起的,查了一下资料,在官方文档的关于自适应哈希索引(https://dev.mysql.com/doc/refman/8.0/en/innodb-adaptive-hash.html)中有下面的信息:

    You can monitor adaptive hash index use and contention in the SEMAPHORES
    section of SHOW ENGINE INNODB STATUS
    output. If there are numerous threads waiting on rw-latches created in btr0sea.c
    , consider increasing the number of adaptive hash index partitions or disabling the adaptive hash index.

    MySQL的错误日志中的确有很多关于btr0sea.cc的锁:

      Last time read locked in file btr0sea.cc line 1037

      使用下面的命令把自适应哈希索引封住:

        mysql> set global innodb_adaptive_hash_index=0;

        然后故障排除。注意还要修改MySQL的配置文件my.cnf,使下次启动后修改继续生效。

        02

        适应哈希索引的适用场景

        在分析了自适应哈希索引的原理后,姚远提醒您在使用这个特性时需要注意下面的问题:

        • 只适用查询语句,并且判断条件是等于,不适用于like和between等非等于的操作符。
        • 对于修改语句因为要维护自适应哈希索引的架构,反而增加了操作的成本。

        自适应哈希索引默认是激活的,因为MySQL最初设计的场景是针对互联网的应用,互联网的应用的特点就是主要是查询操作,很少有修改。因为如果您的应用不是此类应用建议您关闭这个特性,方法是在启动时增加--skip-innodb-adaptive-hash-index这个参数。关于号主,姚远:

        • Oracle ACE(Oracle和MySQL数据库方向)

        • 华为云最有价值专家

        • 《MySQL 8.0运维与优化》的作者

        • 拥有 Oracle 10g、12c和19c OCM等数十项数据库认证

        • 曾任IBM公司数据库部门经理

        • 20+年DBA经验,服务2万+客户

        • 精通C和Java,发明两项计算机专利

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