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Re: Page Checksums + Double Writes

From: Simon Riggs <simon(at)2ndQuadrant(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Kevin Grittner <Kevin(dot)Grittner(at)wicourts(dot)gov>, Alvaro Herrera <alvherre(at)commandprompt(dot)com>, David Fetter <david(at)fetter(dot)org>, Pg Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Page Checksums + Double Writes
Date: 2011-12-22 00:06:49
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Lists: pgsql-hackers
On Wed, Dec 21, 2011 at 11:43 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> It seems like you've forgotten all of the previous discussion of how
> we'd manage a page format version change.

Maybe I've had too much caffeine. It's certainly late here.

> Having two different page formats running around in the system at the
> same time is far from free; in the worst case it means that every single
> piece of code that touches pages has to know about and be prepared to
> cope with both versions.  That's a rather daunting prospect, from a
> coding perspective and even more from a testing perspective.  Maybe
> the issues can be kept localized, but I've seen no analysis done of
> what the impact would be or how we could minimize it.  I do know that
> we considered the idea and mostly rejected it a year or two back.

I'm looking at that now.

My feeling is it probably depends upon how different the formats are,
so given we are discussing a 4 byte addition to the header, it might
be doable.

I'm investing some time on the required analysis.

> A "utility to bump the page version" is equally a whole lot easier said
> than done, given that the new version has more overhead space and thus
> less payload space than the old.  What does it do when the old page is
> too full to be converted?  "Move some data somewhere else" might be
> workable for heap pages, but I'm less sanguine about rearranging indexes
> like that.  At the very least it would imply that the utility has full
> knowledge about every index type in the system.

I agree, rewriting every page is completely out and I never even considered it.

>> I'm now personally game on to make something work here for 9.2.
> If we're going to freeze 9.2 in the spring, I think it's a bit late
> for this sort of work to be just starting.

I agree with that. If this goes adrift it will have to be killed for 9.2.

 Simon Riggs         
 PostgreSQL Development, 24x7 Support, Training & Services

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