Oracle Logbook

April 1, 2014

Over-Configuring DBWR Processes – Part II.

Filed under: Uncategorized — heliodias @ 3:01 pm

Kevin Closson's Blog: Platforms, Databases and Storage

In my recent blog entry about over-configuring DBWR processes, I mentioned the processor cache effect of having too many DBWR processes doing work that a single DBWR process could handle. I’d like to provide more detail on the matter.

Few people really understand what DBWR does for a living. Yes, everyone knows DBWR is responsible for flushing modified buffers from the SGA, but that is really high level. DBWR:

  • 1. Builds “write batches” by examining the state of buffer headers on the LRU lists. It scans the LRU end considering the age of the buffer. At instance boot time, each DBWR process is assigned a set of LRUs to tend to.
  • 2. Manipulates cache buffers chains elements for such reasons as marking the buffer busy when it is in flight for an I/O and marking it as written after the flush operation.
  • 3. Performs I/O using the OSDs…

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