WBFC on Exadata

最新推荐文章于 2021-02-01 15:25:18 发布

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本文探讨了Exadata Write-Back Flash Cache (WBFC) 的使用场景与影响，特别是当数据库未出现“free buffer waits”等待事件时是否启用该特性的考量。WBFC通过缓存脏数据块来减少对Smart Flash Cache的需求，但会占用部分用于正常数据块缓存的空间，存在权衡。

1. "Should we turn on Storage cache as it is not being enabled to further improve the overall performance ?"

Ans:

I guess you are referring to the Exadata Write-Back Flash Cache (WBFC) feature (cache writing of dirty data blocks).

a. One symptom of needing this non-default feature is the db wait event "free buffer waits". We are not having this event. See the end of this email for the reference. Also note that db sessions DO NOT wait directly on the writing of dirty data blocks. Only when there are insufficient free buffers in the db buffer cache, db sessions will be affected. This is why the event is call "free buffer waits".

b. When some Exadata Smart Flash Cache is used to cache the writing of dirty data blocks, less Smart Flash Cache will be used to cache normal data blocks. There is a trade-off.

c. Note that WBFC is NOT for caching the writing of redo blocks. Writing of redo blocks is already cached by another default feature called Flash logging.