Re: 16-bit page checksums for 9.2

From: Bruce Momjian <bruce(at)momjian(dot)us>
To: Martijn van Oosterhout <kleptog(at)svana(dot)org>
Cc: Simon Riggs <simon(at)2ndQuadrant(dot)com>, Andres Freund <andres(at)anarazel(dot)de>,pgsql-hackers(at)postgresql(dot)org, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Subject: Re: 16-bit page checksums for 9.2
Date: 2012-02-05 02:20:56
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On Sun, Dec 25, 2011 at 04:25:19PM +0100, Martijn van Oosterhout wrote:
> On Sat, Dec 24, 2011 at 04:01:02PM +0000, Simon Riggs wrote:
> > On Sat, Dec 24, 2011 at 3:54 PM, Andres Freund <andres(at)anarazel(dot)de> wrote:
> > > Why don't you use the same tricks as the former patch and copy the buffer,
> > > compute the checksum on that, and then write out that copy (you can even do
> > > both at the same time). I have a hard time believing that the additional copy
> > > is more expensive than the locking.
> >
> > ISTM we can't write and copy at the same time because the cheksum is
> > not a trailer field.
> Ofcourse you can. If the checksum is in the trailer field you get the
> nice property that the whole block has a constant checksum. However, if
> you store the checksum elsewhere you just need to change the checking
> algorithm to copy the checksum out, zero those bytes and run the
> checksum and compare with the extracted checksum.
> Not pretty, but I don't think it makes a difference in performence.

Sorry to be late replying to this, but an interesting idea would be to
zero the _hint_ bits before doing the CRC checksum. That would avoid
the problem of WAL-logging the hint bits.

Bruce Momjian <bruce(at)momjian(dot)us>

+ It's impossible for everything to be true. +

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