Re: Enabling Checksums

On 19 March 2013 17:18, Jeff Davis <pgsql@j-davis.com> wrote:

> I will move back to verifying the page hole, as well.

That was agreed long ago...

> There are a few approaches:
>
> 1. Verify that the page hole is zero before write and after read.
> 2. Include it in the calculation (if we think there are some corner
> cases where the hole might not be all zero).
> 3. Zero the page hole before write, and verify that it's zero on read.
> This can be done during the memcpy at no performance penalty in
> PageSetChecksumOnCopy(), but that won't work for
> PageSetChecksumInplace().
>
> With option #2 or #3, we might also verify that the hole is all-zero if
> asserts are enabled.

(3) seems likely to be more expensive than (2), since we're talking
unaligned memory writes rather than a single pre-fetchable block read.

In any case, at initial patch commit, we should CRC the whole block
and allow for the possibility of improvement following measurements.

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