Re: limiting hint bit I/O

On Fri, Jan 14, 2011 at 1:52 PM, Kevin Grittner
<Kevin(dot)Grittner(at)wicourts(dot)gov> wrote:
> Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>
>> Background freezing plays havoc with Hot Standby
>
> I must have missed or forgotten the issue of background vacuums
> and hot standby.  Can you summarize why that's worse than hitting
> thresholds where autovacuum is freezing things?

The critical issue is whether the tuples get frozen while they're
still invisible to some transactions on the standby server. That's
when you get query cancellations.

>> this test is sufficient to show that eliminating hint bits
>> altogether would a significant regression on some workloads.
>
> That wasn't clear to me from what you posted -- I thought that the
> reduced performance might be partly (largely? mostly?) due to
> competition with the background writer's work pushing the hinted
> pages out.  Maybe I'm missing something or you didn't post
> everything you observed in this regard....

Well, let me put together a quick patch that obliterates hint bits
entirely, and we can measure that. The background writer has always
pushed out hint bit pages; I think the reduced performance was
probably due to needing to reset hint bits on pages that we threw away
without pushing them out.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
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