Re: Protecting against unexpected zero-pages: proposal

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@commandprompt.com>
The PostgreSQL Company - Command Prompt, Inc.
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