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Re: [HACKERS] wal_checksum = on (default) | off

From: Ron Mayer <rm_pg(at)cheapcomplexdevices(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Subject: Re: [HACKERS] wal_checksum = on (default) | off
Date: 2007-01-04 19:18:12
Message-ID: 459D52F4.9070704@cheapcomplexdevices.com (view raw or flat)
Thread:
Lists: pgsql-hackerspgsql-patches
Tom Lane wrote:
> Florian Weimer <fweimer(at)bfk(dot)de> writes:
>> Have you tried switching to Adler32 instead of CRC32?
> 
> Is anything known about the error detection capabilities of Adler32?
> There's a lot of math behind CRCs but AFAIR Adler's method is pretty
> much ad-hoc.


As I understand it, it's kinda well studied; but has known
weaknesses in its ability to detect errors under some conditions.

Quoting wikipedia:
"Adler-32 has a weakness for short messages with few hundred bytes,
 because the checksums for these messages have a poor coverage of
 the 32 available bits...Jonathan Stone discovered in 2001 that Adler-32
 has a weakness...An extended explanation can be found in RFC 3309,
 which mandates the use of CRC32 instead of Adler-32...."

I'm not sure if I'm kidding or not here, but I wonder if the not
uncommon requests on the lists of weakening protective features
in postgresql (full-page writes, fsync off, "but mysql says", etc)
suggest that a "dont_protect_against_os_or_hardware_failures" mode
might be in demand for non-critical / development instances.

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