_圖片來自Percona官網_ 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")?對主從數據庫進行校驗，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當前活躍線程數為0，但為什么不繼續執行呢。這個提示信息有點含糊其辭，該工具是用Perl寫的，因此直接打開看腳本跟蹤一下，大概就明白怎么回事了，原來是這個工具有負載保護機制，避免運行時對線上數據庫產生影響。 和這個機制相關的參數名是：?–max-load，其類型是：Array，用法是一個或多個?variables = value?組成的判斷條件，然后根據這個規則判斷某些條件是否超標。例如，設定?–max-load=”Threads_running=25″，意思是當前活躍線程數如果超過25，就暫停 checksum 工作，直到活躍線程數低于 25。 因此，在我們這個案例中，想要強制讓 table-checksum 繼續工作的話，可以設定 –max-load 的值，例如： ~~~ pt-table-checksum --max-load="Threads_running=25" ...其他選項... ~~~ 或者 ~~~ pt-table-checksum --max-load="Threads_connected=25" ...其他選項... ~~~ 前面的選項意思是判斷活躍線程數不要超過25個，后面的選項意思是當前打開的線程數不要超過25個。 下面是 pt-table-checksum 幫助手冊里的一段話： > –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.