Re: Checksums, state of play

On Mon, Mar 5, 2012 at 11:29 PM, Josh Berkus <josh(at)agliodbs(dot)com> wrote:
>
>> 3. Pages with checksums set need to have a version marking to show
>> that they are a later version of the page layout. That version number
>> needs to be extensible to many later versions. Pages of multiple
>> versions need to exist within the server to allow simple upgrades and
>> migration.

> This is a statement of a problem; do you have a proposed solution for it?

Yes. (3) and (4) are in some ways related, so the solution was
presented further down the page.

> (3) and (4) are in conflict with each other, but there is a solution.
> We mark the block with a version number, but we don't make the
> checking dependant upon the version number. We simply avoid making any
> checks until the command to scan all blocks is complete, per point
> (2). That way we need to use 1 flag bit to mark the new version and
> zero flag bits to indicate checks should happen.

The proposed solution is extensible and currently would allow many new versions.

Thank you for your comments.

Are all the other aspects of the proposal acceptable, or are there
other options anybody would like to explore? We've discussed a range
of possible options, so the proposal can be varied as needed.

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