gc current block busy 等待

jiawang

gc current block busy 等待是RAC中global cache全局缓存当前块的争用等待事件， 该等待事件时长由三个部分组成：

Time to process current block request inthe cache= (pin time + flush time + send time)

gc current block flush time

The currentblock flush time is part of the service (or processing) time for a currentblock. The pending redo needs to be flushed to the log file by LGWR before LMSsends it. The operation is asynchronous in that LMS queues the request, postsLGWR, and continues processing. The LMS would check its log flush queue forcompletions and then send the block, or go to sleep and be posted by LGWR. Theredo log write time and redo log sync time can influence theoverall service time significantly.

flush time 是Oracle为了保证Instance Recovery实例恢复机制，而要求每一个current block在本地节点local instance被修改后(modify/update) 必须要将该current block相关的redo 写入到logfile 后（要求LGWR必须完成写入后才能返回)，才能由LMS进程传输给其他节点使用。

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