An event can be anything that Oracle has to perform on behalf of a set of instructions sent by the user interface. The term “wait” is used because every time a user connects to your application, a resource is allocated to perform tasks on its behalf. The waiting comes when a session is waiting for an action, sometimes from a user and at other times from the database

3. gc cr block 2-way :

CR block: A consistent read (CR) block represents a consistent snapshot of the data from a previous point in time. Applying undo/rollback segment information produces consistent read versions.

For example :

We have Instance A and Instance B.
Some foreground process running in Instance A,is trying to access a block using a SELECT statement and that block is not in the local cache of Instance A.To maintain read consistency , foreground process will require the block consistent with the query SCN.
In this case ,operation will performed in following sequence :
1. Foreground session will determine master node of the block,in our case Instance B;and requests a LMS process running in the master node to access the block.
2.And if the master node is the same node having consistent state of block[Means block version is lower or equal to query SCN, then LMS process of Instance B can send the block to the foreground process immediately.

case-1: WRITE/READ

  • Requesting instance request any CR data block for select from Master.
  • If master is the holder of that data block and also has already modified that block.
  • Then master will prepare CR copy of that data block (using undo).
  • Finally Master instance serve CR block to the requesting instance.
  • Meanwhile requesting instace will wait in “gc cr block 2-way”

Case-2: READ/READ

  • Requesting instance request any CR data block for select from Master.
  • If master is the holder of that data block and has not already modified that block.
  • Master instance serve CR block to the requesting instance immediatly.
  • Meanwhile requesting instace will wait in “gc cr block 2-way”

4. gc cr block 3-way :

For example :

We have Instance A,Instance B and Instance C:
1.One foreground process running in Instance A,is trying to access a block using SELECT statement and that block is not in the local cache of Instance A.
2.Foreground session will determine master node of the block , Instance B. when Instance B is resource master for that block  is not having consistent image of the block compared to query SCN.
3.Then master Instance[Instance B], forwards the request to holding Instance – Instance C , making it more understandable , lets say requested block has an uncommitted changes.
4.Since the block has uncommitted changes,LMS process can not send block immediately.LMS process creates a CR version of the block by cloning the buffer and applying undo records to the buffer rolling back the block to the SCN consistent with the requested query SCN.
5.Then CR [consistent read] block is sent to the foreground process.

Basic Solution to resolve above two wait events

  • Best Solution is to apply application Segregation means try to locate all select query on one node and all DML on another node.
  • Tune LGWR
  • Tune Interconnect

Stay Tuned for more detail on Oracle RAC wait events.

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