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Re: Block-level CRC checks

From: Alvaro Herrera <alvherre(at)commandprompt(dot)com>
To: Gregory Stark <stark(at)enterprisedb(dot)com>
Cc: Martijn van Oosterhout <kleptog(at)svana(dot)org>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Greg Stark <greg(dot)stark(at)enterprisedb(dot)com>, Pg Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Block-level CRC checks
Date: 2008-11-13 18:00:00
Message-ID: (view raw or whole thread)
Lists: pgsql-hackers
Gregory Stark wrote:

> I think we're talking past each other. Martin and I are talking about doing
> something like:
> for (...)
>   ...
>   crc(word including hint bits)
>   ...
> for (each line pointer)
>   crc-negated(word & LP_DEAD<<15)
> Because CRC is a cyclic checksum it's possible to add or remove bits
> incrementally.

I see.

Since our CRC implementation is a simple byte loop, and since ItemIdData
fits in a uint32, the attached patch should do mostly the same by
copying the line pointer into a uint32, turning off the lp_flags, and
summing the modified copy.

This patch is also skipping pd_special and the unused area of the page.

I'm still testing this; please beware that this likely has an even
higher bug density than my regular patches (and some debugging printouts
as well).

While reading the pg_filedump code I noticed that there's a way to tell
the different index pages apart, so perhaps we can use that to be able
to checksum the special space as well.

Alvaro Herrera                      
PostgreSQL Replication, Consulting, Custom Development, 24x7 support

Attachment: blkcrc-11.patch
Description: text/x-diff (27.0 KB)

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