Re: 16-bit page checksums for 9.2

On Mon, Feb 6, 2012 at 4:48 AM, Bruce Momjian <bruce(at)momjian(dot)us> wrote:
> On Sun, Feb 05, 2012 at 08:40:09PM +0000, Simon Riggs wrote:
>> 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.
>
> Thanks.  Clearly we don't need 16 bits to represent our page version
> number because we rarely change it.  The other good thing is I don't
> remember anyone wanting additional per-page storage in the past few
> years except for a checksum.
>
> I wonder if we should just dedicate 3 page header bits, call that the
> page version number, and set this new version number to 1, and assume
> all previous versions were zero, and have them look in the old page
> version location if the new version number is zero.  I am basically
> thinking of how we can plan ahead to move the version number to a new
> location and have a defined way of finding the page version number using
> old and new schemes.
>
> I don't want to get to a point where we need a bit per page number
> version.

Agreed, though I think we need at least 2 bits set if we are using the
checksum, so we should start at version 2 to ensure that.

--
 Simon Riggs                   http://www.2ndQuadrant.com/
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