Miscellaneous Oracle RAC wait events

1. gc remaster :
resource remastering happens based on 2 situation :
1.When Instance leaves or joins cluster.
2.Dynamic remastering from a less busy instance to an instance where demand for that object is too high.This is also called resource affinity.

2. gc cr grant busy :
The gc cr grant busy event is a when a grant is received but there is a delay in loading the block This can be because there are many versions of the data block in the RAM buffers of the RAC database. The “gc cr grant busy” wait can also occur when using the insert with the APPEND hint, as Oracle may be busy formatting block headers and footers for a new insert.

3.gc cr grant congested :
These wait events indicate that there were
CPU resource
starvation or higher global cache workload
For example, sudden spikes in global cache workload,
CPU/memory starvation etc.
Reducing CPU usage by tuning costly SQL statement.

4. gc cr multi block request :
This event happends when using full table scan.
Reducing full table scan will reduce the time spent on this event.

5.gc freelist wait :

Locks are not being closed fast enough ,resulting in gc freelist waits.
All releasable locks are used and a new one has been requested . To make a resource element available , a resource element is pinged.

Wait time : The duration of the resource get operation to ping the resource element.

6. gc current retry :Instance asks for a current block , and if requested block was not received either because , the block was corrupted or the block was lost during transmission over the interconnect .

7. gc cr failure: This is similar to the gc current retry event, but in this case, CR block was requested. However, a failure status

was received either because the block was corrupted or the block was lost during transmission over the interconnect.
8.Wait for master scn :
Each node in the cluster generates its own SCN and subsequently , using propagation method resynchronizes to the highest SCN in the cluster.
This wait indicates number of time foreground process waited for SCNs to be acknowledged from instances in the cluster.
This function is driven by underscore [hidden] parameter “_IMMEDIATE_COMMIT_PROPAGATION” having boolean value true or false.
When it is set to TRUE oracle uses the Block on Commit algorithm for messaging.In this case of BOC ,global high water mark for the SCNS sent and received is maintained, there by reducing traffic for global SCN synchronization and in turn improving overall performance.
9.gc disk read :LMS is a critical process and does not do heavy lifting tasks such as disk I/O etc. If LMS process has to initiate I/O, instead of initiating I/O, LMS will downgrade the block mode and send the block to the requesting foreground process (this is known as Light Works rule). Foreground process will apply undo records to the block to construct CR version of the block.

Now, the Foreground process might not find the undo blocks in the local cache as the transactions happened in the remote cache. A request is sent to remote LMS process to access undo block. If the undo block is not in the remote cache either, remote LMS process will send a grant to the foreground process to read the undo block from the disk. Foreground process accounts this wait time for the undo segment block grants to the gc cr disk read wait event.

10.gc cr/current block lost :

gc cr/current block can get lost during transmission in interconnect.
If a requested block is not received in the instance in 0.5 second , the block is considered to be lost block. And this block need to be resent , means wasting time for second transfer to complete.

11.gc cr/current cancel 

12.gc current split

13.gc prepare

14.gc claim

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