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Re: 16-bit page checksums for 9.2

From: Simon Riggs <simon(at)2ndQuadrant(dot)com>
To: Bruce Momjian <bruce(at)momjian(dot)us>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: 16-bit page checksums for 9.2
Date: 2012-02-05 20:40:09
Message-ID: CA+U5nMJkNuCa26P5mPr=Cb_YftkRgzt+WziQHSQQm-Uhz0g=8A@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackers
On Sun, Feb 5, 2012 at 3:59 AM, Bruce Momjian <bruce(at)momjian(dot)us> wrote:
> On Sat, Dec 24, 2011 at 03:56:58PM +0000, Simon Riggs wrote:
>> > Also, as far as I can see this patch usurps the page version field,
>> > which I find unacceptably short-sighted.  Do you really think this is
>> > the last page layout change we'll ever make?
>>
>> No, I don't. I hope and expect the next page layout change to
>> reintroduce such a field.
>>
>> But since we're agreed now that upgrading is important, changing page
>> format isn't likely to be happening until we get an online upgrade
>> process. So future changes are much less likely. If they do happen, we
>> have some flag bits spare that can be used to indicate later versions.
>> It's not the prettiest thing in the world, but it's a small ugliness
>> in return for an important feature. If there was a way without that, I
>> would have chosen it.
>
> Have you considered the CRC might match a valuid page version number?
> Is that safe?

In the proposed scheme there are two flag bits set on the page to
indicate whether the field is used as a checksum rather than a version
number. So its possible the checksum could look like a valid page
version number, but we'd still be able to tell the difference.

-- 
 Simon Riggs                   http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services

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