How Oracle RAC Cache Fusion Works

Learn the mysteries of how Oracle RAC Cache Fusion works. Cache Fusion is the secret to making Oracle RAC a scalable database platform. You will learn how cache fusion works, the differences between global cache current and consistent read transfers, the differences between two-way and three-way transfers and the associated wait events.

Oracle Ace and author Brian Peasland provides an in-depth demonstration of global cache transfers in action by utilizing Oracle Trace and session statistics (v$sesstat and v$statname).

Prerequisites: Knowledge of Oracle Architecture including the concept of an Oracle instance, the buffer cache, how data is moved into the buffer cache and an understanding of wait events.

Brian is a certified Oracle ACE who has been in the IT field for over 25 years and has worked as a computer operator, operations analyst, systems administrator, application developer and for more than half of his career as a database administrator. He holds a B.S. in Computer Science and a M.S. in Computer Science, specializing in database systems. Brian has been a member of’s Ask The Experts since 2001 and now contributes articles to the site. He regularly contributes to the My Oracle Support and Oracle Technet communities. He can be followed on Twitter on @BPeaslandDBA and maintains a blog at

Buy Brian’s new book “Oracle RAC Performance Tuning” at Amazon.

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This free training is segmented into several separate lessons:

  1. Lesson 1 – Agenda (6:02)
    Brian’s introduction and tutorial agenda plus a brief history of Oracle clustering (parallel server) and the problems associated with disk pinging.
  2. Lesson 2 – Demonstration: Cache Fusion in Action (5:20)
    Learn how Oracle RAC Cache Fusion really works. Brian Peasland demonstrates, with v$statname, v$sesstat and Oracle trace (dbms_monitor.session_trace_enable) cache fusion in action.
  3. Lesson 3 – Demonstration Understanding Global Cache Statistics and Trace Output (9:13)
    A continuation of the cache fusion demonstration, with a detailed look at session statistics and the raw trace file contents. You will learn about the “gc” wait events (Global Cache) including ‘gc current block 2-way’ and ‘gc cr multi block request’.
  4. Lesson 4 – demonstration: Global Cache Current Transfer vs Consistent Read Transfer (4:38)
    Learn the difference between a current block and a consistent read block.
  5. Lesson 5 – 2-Way vs 3-Way Block Transfers (4:07)
    Brian demonstrates 2-Way vs 3-Way Block Transfers including a description of master, holders and requestors.
  6. (2:03) (click on video below)
    Oracle ACE and author Brian Peasland provides a summary of the “How Oracle RAC Cache Fusion Works” tutorial.

Date: Oct 19, 2015

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How Oracle RAC Cache Fusion Really Works Part 6 of 6


>> Brian:  The three-way transfers. Now these wait event names which may have seems cryptic at first should make so much more sense. 


To recap, we showed current global cache transfer which are nothing more than a session obtaining the block via Cache Fusion from another instance’s buffer cache and that block is getting as the most recent that contains no uncommitted transactions. Then we also saw an example where we had a session in one instance with an uncommitted transaction and a session in another instance that was requesting the block to be transferred via Cache Fusion. So Oracle had to generate a consistent read image of that block. Hence, the CR wait event. 


Our examples were only on a two node RAC cluster in a virtual environment on my laptop here, so therefore we could only see some two-way wait events. If there were three or more instances involved, we could’ve seen three-way wait events. Remember that the three instances involved could be some combination of the resource master, the holding instance of the block, and the instance requesting that block. 




I appreciate you watching my video. I hope you found it informational and instructive. Thank you for your time.


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