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

From: "Simon Riggs" <simon(at)2ndquadrant(dot)com>
To: "Tom Lane" <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: <pgsql-hackers(at)postgresql(dot)org>, <pgsql-patches(at)postgresql(dot)org>
Subject: Re: [HACKERS] wal_checksum = on (default) | off
Date: 2007-01-04 16:51:50
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On Thu, 2007-01-04 at 11:09 -0500, Tom Lane wrote:
> "Simon Riggs" <simon(at)2ndquadrant(dot)com> writes:
> > On Thu, 2007-01-04 at 10:00 -0500, Tom Lane wrote:
> >> "Simon Riggs" <simon(at)2ndquadrant(dot)com> writes:
> >>> Recovery can occur with/without same setting of wal_checksum, to avoid
> >>> complications from crashes immediately after turning GUC on.
> >>
> >> Surely not. Otherwise even the "on" setting is not really a defense.
> > Only when the CRC is exactly zero, which happens very very rarely.
> "It works most of the time" doesn't exactly satisfy me. What's the
> use-case for changing the variable on the fly anyway? Seems a better
> solution is just to lock down the setting at postmaster start.

That would prevent us from using the secondary checkpoint location, in
the case of a crash effecting the primary checkpoint when it is a
shutdown checkpoint where we changed the setting of wal_checksum. It
seemed safer to allow a very rare error through to the next level of
error checking rather than to close the door so tight that recovery
would not be possible in a very rare case.

If your're good with server start, so am I.

Simon Riggs

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