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List:       pgsql-hackers
Subject:    Re: [HACKERS] 16-bit page checksums for 9.2
From:       Jim Nasby <jim () nasby ! net>
Date:       2012-02-29 23:38:24
Message-ID: 4F4EB6F0.3060706 () nasby ! net
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On 2/29/12 3:53 PM, Alvaro Herrera wrote:
> 
> Excerpts from Tom Lane's message of mié feb 29 18:34:27 -0300 2012:
> > 
> > Robert Haas<robertmhaas@gmail.com>  writes:
> > > On Wed, Feb 29, 2012 at 2:33 PM, Heikki Linnakangas
> > > <heikki.linnakangas@enterprisedb.com>  wrote:
> > > > The utility would run in the old cluster before upgrading, so the the flag
> > > > would have to be present in the old version. pg_upgrade would check that the
> > > > flag is set, refusing to upgrade if it isn't, with an error like "please run
> > > > pre-upgrade utility first".
> > 
> > > I find that a pretty unappealing design; it seems to me it'd be much
> > > easier to make the new cluster cope with everything.
> > 
> > Easier for who?  I don't care for the idea of code that has to cope with
> > two page formats, or before long N page formats, because if we don't
> > have some mechanism like this then we will never be able to decide that
> > an old data format is safely dead.
> 
> .. in fact this is precisely what killed Zdenek Kotala's idea of
> upgrading.

This is also why Simon has avoided the whole upgrade thing with his 16 bit checksum \
idea (otherwise presumably we'd be looking at bigger checksums anyway).

I get that fussing around with the version field is ugly. If there was another way to \
do this without breaking pg_upgrade then it would be silly to mess with the version \
field. Unfortunately, there is no other way.

Page checksuming is something a lot of people (myself included) want to see. Being \
able to get it in 9.2 would be a big win over crossing our fingers that at some point \
in the future (who knows when) we'll maybe figure out the page format upgrade issue. \
While we should definitely be looking into that issue it's definitely not going to \
get fixed in 9.2. ISTM that checksums are actually ready to go if people can just \
swallow the bitter pill of a screwed-up page version field until we can actually get \
an upgrade utility in place (and until we get that utility in place I don't see that \
                the version field would do us any good anyway...)
-- 
Jim C. Nasby, Database Architect                   jim@nasby.net
512.569.9461 (cell)                         http://jim.nasby.net

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