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Re: Block-level CRC checks

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Bruce Momjian <bruce(at)momjian(dot)us>
Cc: Simon Riggs <simon(at)2ndQuadrant(dot)com>, Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>, Aidan Van Dyk <aidan(at)highrise(dot)ca>, Alvaro Herrera <alvherre(at)commandprompt(dot)com>, Pg Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Block-level CRC checks
Date: 2009-12-01 21:56:49
Message-ID: 22679.1259704609@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
Bruce Momjian <bruce(at)momjian(dot)us> writes:
> OK, crazy idea #3.  What if we had a per-page counter of the number of
> hint bits set --- that way, we would only consider a CRC check failure
> to be corruption if the count matched the hint bit count on the page.

Seems like rather a large hole in the ability to detect corruption.
In particular, this again assumes that you can accurately locate all
the hint bits in a page whose condition is questionable.  Pick up the
wrong bits, you'll come to the wrong conclusion --- and the default
behavior you propose here is the wrong result.

			regards, tom lane

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