gc current block 3-way

gc current block 3-way (write/write with 3 nodes)

Definition

- Requesting instance request any data block in CURRENT MODE for dml(current) from Master.
- If master is not holder of that data block and that data block is globally available on another instance.
- Master will send a message to the current holding instance to relinquish ownership (Downgrade lock).
- The holding instance retain the PI of that data block and then serve to the requesting instance.
- Holding instance will also flush respective redo log to log file before sending CURRENT block to requesting instance.
- Meanwhile requesting instance will wait in "GC CURRENT BLOCK 3-WAY"


Reason

As you know before sending the current data block to the requesting instance master instance first flush respective redo log of the log file then it will prepare PI block and then CR block to send to the requesting instance over the interconnect.

- This wait event appears in "TOP-5" timed events section of AWR Report.
- Analyze the contention using AWR REPORT.
- In AWR REPORT, analyze "Current Block Recieved" and identify the top contentious objects.
- You are getting contention at segment level because of bad database design, DB object layout and Space Management.
- LGWR is not efficient to write in redo log file and thats why requesting instance is waiting in "gc current block 3-way".
- Interconnect is having N/W latency.
- Application Segregation is also a reason.


Troubleshooting

- 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