enq: WF - contention等待事件

前幾天在客戶現場看到一個比較少見的資料庫等待事件  enq: WF - contention
下面是找到的關於這個等待事件的一些描述

What is the "WF - Contention'' Enqueue ?

MMON (Manageability Monitor) is attempting the autoflush function which means that we are trying to flush the ASH (Active Session History) data into the AWR (Active Workload Repository) for the historical repository. 

We take out the WF enqueue for doing this and hence in your wait sessions you may see WF enqueue contention.

What is the "WF - Contention'' Enqueue ? (Doc ID 358208.1)

也就是說，MMON程式在將ASH的資料匯入到AWR中時，會請求一個WF的enqueue。這個等待事件一般很少出現，也很少會在AWR報告中的top 5中展現出來。
當天客戶出現這個等待事件期間，RAC叢集的儲存鏈路出現了問題。除了這個等待事件，資料庫中還有大量的Disk IO等待。因此我們大概可以推斷是因為在儲存鏈路出現問題時，剛好ash資料往awr中寫。導致這個等待事件比較明顯。（該資料庫本身平時的壓力並不大）

下面是網上找到的Jonathan Lewis對這個等待事件的描述：
As far as my understanding goes it shouldn't matter and shouldn't be reported in the Top 5 anyway). This is just one instance "saying" - is it safe for me to flush my AWR (and ASH) data to disc without causing cross instance contention - and the instances are synchhronised through this lock. But if you have a very large number of active sessions, or the SGA is small you could find that the ASH flush happens more frequently that once per hour because there isn't enough memory allocated to ASH to hold an hour's worth of data.

https://community.oracle.com/thread/2180172

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