From: | Alvaro Herrera <alvherre(at)commandprompt(dot)com> |
---|---|
To: | Robert Haas <robertmhaas(at)gmail(dot)com> |
Cc: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Russell Smith <mr-russ(at)pws(dot)com(dot)au>, Josh Berkus <josh(at)agliodbs(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: Idea for getting rid of VACUUM FREEZE on cold pages |
Date: | 2010-06-02 19:10:23 |
Message-ID: | 1275505502-sup-6468@alvh.no-ip.org |
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Lists: | pgsql-hackers |
Excerpts from Robert Haas's message of mié jun 02 14:16:33 -0400 2010:
> We could, but I think we'd be better off just freezing at the time we
> mark the page PD_ALL_VISIBLE and then using the visibility map for
> both purposes. Keeping around the old xmin values after every tuple
> on the page is visible to every running transaction is useful only for
> forensics, and building a whole new freeze map just to retain that
> information longer (and eventually force a massive anti-wraparound
> vacuum) seems like overkill.
Reducing the xid wraparound horizon "a bit" is reasonable, but moving it
all the way forward to OldestXmin is a bit much, methinks.
Besides, there's another argument for not freezing tuples immediately:
they may be updated shortly thereafter, causing extra churn for no gain.
I'd prefer a setting that would tell the system to freeze all tuples
that fall within a safety range whenever any tuple in the page is frozen
-- weren't you working on a patch to do this? (was it Jeff Davis?)
(BTW maybe instead of separate visibility and freeze maps we could have
two bits in the visibility map?)
--
Álvaro Herrera <alvherre(at)commandprompt(dot)com>
The PostgreSQL Company - Command Prompt, Inc.
PostgreSQL Replication, Consulting, Custom Development, 24x7 support
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