_图片来自Percona官网_ 今天同事在用 [percona toolkit](http://www.percona.com/software/percona-toolkit "percona toolkit") 工具中的 [pt-table-checksum](http://www.percona.com/doc/percona-toolkit/2.2/pt-table-checksum.html "pt-table-checksum") 对主从数据库进行校验，提交命令后，一直提示下面的信息： ~~~ Pausing because Threads_running=0 ~~~ 看字面意思是在提示当前活跃线程数为0，但为什么不继续执行呢。这个提示信息有点含糊其辞，该工具是用Perl写的，因此直接打开看脚本跟踪一下，大概就明白怎么回事了，原来是这个工具有负载保护机制，避免运行时对线上数据库产生影响。 和这个机制相关的参数名是： –max-load，其类型是：Array，用法是一个或多个 variables = value 组成的判断条件，然后根据这个规则判断某些条件是否超标。例如，设定 –max-load=”Threads_running=25″，意思是当前活跃线程数如果超过25，就暂停 checksum 工作，直到活跃线程数低于 25。 因此，在我们这个案例中，想要强制让 table-checksum 继续工作的话，可以设定 –max-load 的值，例如： ~~~ pt-table-checksum --max-load="Threads_running=25" ...其他选项... ~~~ 或者 ~~~ pt-table-checksum --max-load="Threads_connected=25" ...其他选项... ~~~ 前面的选项意思是判断活跃线程数不要超过25个，后面的选项意思是当前打开的线程数不要超过25个。 下面是 pt-table-checksum 帮助手册里的一段话： > –max-load > type: Array; default: Threads_running=25; group: Throttle > > Examine SHOW GLOBAL STATUS after every chunk, and pause if any status variables are higher than the threshold. The option accepts a comma-sep- > arated list of MySQL status variables to check for a threshold. An optional “=MAX_VALUE” (or “:MAX_VALUE”) can follow each variable. If not > given, the tool determines a threshold by examining the current value and increasing it by 20%. > > For example, if you want the tool to pause when Threads_connected gets too high, you can specify “Threads_connected”, and the tool will check > the current value when it starts working and add 20% to that value. If the current value is 100, then the tool will pause when Threads_con- > nected exceeds 120, and resume working when it is below 120 again. If you want to specify an explicit threshold, such as 110, you can use > either “Threads_connected:110″ or “Threads_connected=110″. > > The purpose of this option is to prevent the tool from adding too much load to the server. If the checksum queries are intrusive, or if they > cause lock waits, then other queries on the server will tend to block and queue. This will typically cause Threads_running to increase, and the > tool can detect that by running SHOW GLOBAL STATUS immediately after each checksum query finishes. If you specify a threshold for this vari- > able, then you can instruct the tool to wait until queries are running normally again. This will not prevent queueing, however; it will only > give the server a chance to recover from the queueing. If you notice queueing, it is best to decrease the chunk time.