通常情况下，存储引擎会设置自己的每个线程的全局分配内存，它通常不能像缓存一样可以调节。

In general storage engines may have their own per thread or global memory allocations which are not tuned as buffers.

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在设置为after_commit参数，发起事务的客户端仅在服务器向存储引擎写入数据并接受从库得到确认之后才返回状态。

With AFTER_COMMIT, the client issuing the transaction gets a return status only after the server commits to the storage engine and receives slave acknowledgment.

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