On 8/24/2007 8:41 AM, Heikki Linnakangas wrote:
If anyone out there has a repeatable test case where bgwriter does help,
I'm all ears. The theory of moving the writes out of the critical path
does sound reasonable, so I'm sure there is test case to demonstrate the
effect, but it seems to be pretty darn hard to find.


One could try to dust off this TPC-W benchmark.

    http://pgfoundry.org/projects/tpc-w-php/

Again, the original theory for the bgwriter wasn't moving writes out of the critical path, but smoothing responsetimes that tended to go completely down the toilet during checkpointing, causing all the users to wake up and overload the system entirely.

It is well known that any kind of bgwriter configuration other than OFF does increase the total IO cost. But you will find that everyone who has SLA's that define maximum response times will happily increase the IO bandwidth to give an aggressively configured bgwriter room to work.


Jan

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