Mysql(MyISAM)的读写互斥锁有关问题的解决办法
Mysql(MyISAM)的读写互斥锁问题的解决方法 最近因为数据库读的请求增加,出现了比较严重的读写锁问题,由于主从分离,主服务器很快的执行完了写入的操作,但从库由于有大量的select的查询,会被这些来自主辅同步的update,insert严重堵塞,最后造成所有的Mysql
Mysql(MyISAM)的读写互斥锁问题的解决方法
最近因为数据库读的请求增加,出现了比较严重的读写锁问题,由于主从分离,主服务器很快的执行完了写入的操作,但从库由于有大量的select的查询,会被这些来自主辅同步的update,insert严重堵塞,最后造成所有的Mysql从库负载迅速上升。
由于没办法在短期内增加读的服务器,所以采取对Mysql进行了一些配置,以牺牲数据实时性为代价,来换取所有服务器的生命安全。呵呵,具体相关调整以及思路如下:
MyISAM在读操作占主导的情况下是很高效的。可一旦出现大量的读写并发,同InnoDB相比,MyISAM的效率就会直线下降,而且,MyISAM和 InnoDB的数据存储方式也有显著不同:通常,在MyISAM里,新数据会被附加到数据文件的结尾,可如果时常做一些UPDATE,DELETE操作之后,数据文件就不再是连续的,形象一点来说,就是数据文件里出现了很多洞洞,此时再插入新数据时,按缺省设置会先看这些洞洞的大小是否可以容纳下新数据,如果可以,则直接把新数据保存到洞洞里,反之,则把新数据保存到数据文件的结尾。之所以这样做是为了减少数据文件的大小,降低文件碎片的产生。但 InnoDB里则不是这样,在InnoDB里,由于主键是cluster的,所以,数据文件始终是按照主键排序的,如果使用自增ID做主键,则新数据始终是位于数据文件的结尾。
了解了这些基础知识,下面说说MyISAM几个容易忽视的配置选项:
concurrent_insert:
通常来说,在MyISAM里读写操作是串行的,但当对同一个表进行查询和插入操作时,为了降低锁竞争的频率,根据concurrent_insert的设置,MyISAM是可以并行处理查询和插入的:
当concurrent_insert=0时,不允许并发插入功能。
当concurrent_insert=1时,允许对没有洞洞的表使用并发插入,新数据位于数据文件结尾(缺省)。
当concurrent_insert=2时,不管表有没有洞洞,都允许在数据文件结尾并发插入。
这样看来,把concurrent_insert设置为2是很划算的,至于由此产生的文件碎片,可以定期使用OPTIMIZE TABLE语法优化。
max_write_lock_count:
缺省情况下,写操作的优先级要高于读操作的优先级,即便是先发送的读请求,后发送的写请求,此时也会优先处理写请求,然后再处理读请求。这就造成一个问题:一旦我发出若干个写请求,就会堵塞所有的读请求,直到写请求全都处理完,才有机会处理读请求。此时可以考虑使用max_write_lock_count:
max_write_lock_count=1
有了这样的设置,当系统处理一个写操作后,就会暂停写操作,给读操作执行的机会。
low-priority-updates:
我们还可以更干脆点,直接降低写操作的优先级,给读操作更高的优先级。
low-priority-updates=1
综合来看,concurrent_insert=2是绝对推荐的,至于max_write_lock_count=1和low-priority-updates=1,则视情况而定,如果可以降低写操作的优先级,则使用low-priority-updates=1,否则使用max_write_lock_count=1。
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