Re: Enabling Checksums

On 17 December 2012 19:29, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Simon Riggs <simon@2ndQuadrant.com> writes:
>> Discussing this makes me realise that we need a more useful response
>> than just "your data is corrupt", so user can respond "yes, I know,
>> I'm trying to save whats left".
>
>> We'll need a way of expressing some form of corruption tolerance.
>> zero_damaged_pages is just insane, much better if we set
>> corruption_tolerance = N to allow us to skip N corrupt pages before
>> failing, with -1 meaning keep skipping for ever. Settable by superuser
>> only.
>
> Define "skip".

Allow data access, but accept that the answer is silently incomplete.
Not really much difference from zero_damaged_pages which just removes
the error by removing any chance of repair or recovery, and then
silently gives the wrong answer.

> Extra points if it makes sense for an index.

I guess not, but that's no barrier to it working on heap pages only,
in my suggested use case.

> And what about things like pg_clog pages?

SLRUs aren't checksummed because of their lack of header space.
Perhaps that is a major point against the patch.

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