From: | Alvaro Herrera <alvherre(at)commandprompt(dot)com> |
---|---|
To: | Robert Haas <robertmhaas(at)gmail(dot)com> |
Cc: | Greg Stark <gsstark(at)mit(dot)edu>, Aidan Van Dyk <aidan(at)highrise(dot)ca>, Jim Nasby <jim(at)nasby(dot)net>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Gurjeet Singh <singh(dot)gurjeet(at)gmail(dot)com>, PGSQL Hackers <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: Protecting against unexpected zero-pages: proposal |
Date: | 2010-11-09 19:15:01 |
Message-ID: | 1289329940-sup-5862@alvh.no-ip.org |
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Thread: | |
Lists: | pgsql-hackers |
Excerpts from Robert Haas's message of mar nov 09 16:05:57 -0300 2010:
> And it still allows silent data corruption, because bogusly clearing a
> hint bit is, at the moment, harmless, but bogusly setting one is not.
> I really have to wonder how other products handle this. PostgreSQL
> isn't the only database product that uses MVCC - not by a long shot -
> and the problem of detecting whether an XID is visible to the current
> snapshot can't be ours alone. So what do other people do about this?
> They either don't cache the information about whether the XID is
> committed in-page (in which case, are they just slower or do they have
> some other means of avoiding the performance hit?) or they cache it in
> the page (in which case, they either WAL log it or they don't checksum
> it). I mean, there aren't any other options, are there?
Maybe allocate enough shared memory for pg_clog buffers back to the
freeze horizon, and just don't use hint bits? Maybe some intermediate
solution, i.e. allocate a large bunch of pg_clog buffers, and do
WAL-logged setting of hint bits only for tuples that go further back.
I remember someone had a patch to set all the bits in a page that passed
a threshold of some kind. Ah, no, that was for freezing tuples.
--
Álvaro Herrera <alvherre(at)commandprompt(dot)com>
The PostgreSQL Company - Command Prompt, Inc.
PostgreSQL Replication, Consulting, Custom Development, 24x7 support
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