Re: WAL record CRC calculated incorrectly because of underlying buffer modification

From: Jeff Davis <pgsql(at)j-davis(dot)com>
To: Michael Paquier <michael(at)paquier(dot)xyz>
Cc: Thomas Munro <thomas(dot)munro(at)gmail(dot)com>, Alexander Lakhin <exclusion(at)gmail(dot)com>, Andres Freund <andres(at)anarazel(dot)de>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>, Heikki Linnakangas <hlinnaka(at)iki(dot)fi>
Subject: Re: WAL record CRC calculated incorrectly because of underlying buffer modification
Date: 2024-05-17 14:56:23
Message-ID: a4e1196a77f154ccf8df486a632d998525a1becc.camel@j-davis.com
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On Fri, 2024-05-17 at 10:12 +0900, Michael Paquier wrote:
> That's something I've done four weeks ago in the hash replay code
> path, and having the image with XLR_CHECK_CONSISTENCY even if
> REGBUF_NO_CHANGE was necessary because replay was setting up a LSN on
> a REGBUF_NO_CHANGE page it should not have touched.

Then the candidate fix to selectively break XLR_CHECK_CONSISTENCY is
not acceptable.

>
> Yeah, agreed that getting rid of REGBUF_NO_CHANGE would be nice in
> the
> final picture.  It still strikes me as a weird concept that WAL
> replay
> for hash indexes logs full pages just to be able to lock them at
> replay based on what's in the records.  :/

I'm still not entirely clear on why hash indexes can't just follow the
rules and exclusive lock the buffer and dirty it. Presumably
performance would suffer, but I asked that question previously and
didn't get an answer:

https://www.postgresql.org/message-id/CA%2BTgmoY%2BdagCyrMKau7UQeQU6w4LuVEu%2ByjsmJBoXKAo6XbUUA%40mail.gmail.com

And if that does affect performance, what about following the same
protocol as XLogSaveBufferForHint()?

Regards,
Jeff Davis

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