> I have tuned the checkpoint_timeout to 30 second which is ten times less > than default and the issue is still reproduced.
Doing a checkpoint every 30 seconds is crazy; no wonder your system is pausing so much. Put the timeout back to the default. What you should do here is edit your config file and set checkpoint_warning to its maximum of 3600. After that, take a look at the log files; you'll then get a warning message every time a checkpoint happens. If those line up with when you're getting the slowdowns, then at least you'll have narrowed the cause of your problem, and you can get some advice here on how to make the overhead of checkpoints less painful.
The hint it will give is probably the first thing to try: increase checkpoint_segments from the default to something much larger (if it's at 3 now, try 10 instead to start), and see if the problem stops happening as frequently. Your problem looks exactly like a pause at every checkpoint, and I'm not sure what Richard was thinking when he suggested having them more often would improve things.
Yes, Thank you for your suggestion. i have found that the slowdown time does not align to checkpoint after i turned on the warning. The issue is related what Richard has been mentioned - Something outsides PG doing many write operations to pages.
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