| From: | Alvaro Herrera <alvherre(at)2ndquadrant(dot)com> |
|---|---|
| To: | Jeff Janes <jeff(dot)janes(at)gmail(dot)com> |
| Cc: | Andres Freund <andres(at)2ndquadrant(dot)com>, Josh Berkus <josh(at)agliodbs(dot)com>, Jim Nasby <Jim(dot)Nasby(at)bluetreble(dot)com>, Bruce Momjian <bruce(at)momjian(dot)us>, Alexey Bashtanov <bashtanov(at)imap(dot)cc>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Autovacuum fails to keep visibility map up-to-date in mostly-insert-only-tables |
| Date: | 2014-10-27 21:51:44 |
| Message-ID: | 20141027215144.GX1791@alvin.alvh.no-ip.org |
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| Lists: | pgsql-hackers |
Jeff Janes wrote:
> It is only a page read if you have to read the page. It would seem optimal
> to have bgwriter adventitiously set hint bits and vm bits, because that is
> the last point at which the page can be changed without risking that it be
> written out twice. At that point, it has been given the maximum amount of
> time it can be given for the interested transactions to have committed and
> to have aged past the xmin horizon. I seem to recall that the main problem
> with that, though, is that you must be attached to a database in order to
> determine visibility, and bgwriter is not attached to a database.
Regarding tuple hint bits, I couldn't find any such limitation in
SetHintBits, other than in MarkBufferDirtyHint there being some code
that would cause trouble: it accesses MyPgXact, which bgwriter would
obviously not have. Maybe worth some experimentation ...
I'm not sure about vm bits, though. That's a whole different topic.
--
Álvaro Herrera http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services
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