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
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
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.
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
Thank you for giving your valuable time to read the above information.
If you want to be updated with all our articles send us the Invitation or Follow us:
Skant Gupta’s LinkedIn: www.linkedin.com/in/skantali/
Joel Perez’s LinkedIn: Joel Perez’s Profile
Anuradha’s LinkedIn: Anuradha’s Profile
LinkedIn Group: Oracle Cloud DBAAS
Facebook Page: OracleHelp