Re: Block-level CRC checks

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 http://www.CommandPrompt.com/
PostgreSQL Replication, Consulting, Custom Development, 24x7 support