Re: Remove secondary checkpoint

On 24 October 2017 at 09:50, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Simon Riggs <simon(at)2ndquadrant(dot)com> writes:
>> Remove the code that maintained two checkpoint's WAL files and all
>> associated stuff.
>
>> Try to avoid breaking anything else
>
>> This
>> * reduces disk space requirements on master
>> * removes a minor bug in fast failover
>> * simplifies code
>
> Doesn't it also make crash recovery less robust? The whole point
> of that mechanism is to be able to cope if the latest checkpoint
> record is unreadable. If you want to toss that overboard, I think
> you need to make the case why we don't need it, not just post a
> patch removing it. *Of course* the code is simpler without it.
> That's utterly irrelevant. The code would be even simpler with
> no crash recovery at all ... but we're not going there.

Well, the mechanism has already been partially removed since we don't
maintain two checkpoints on a standby. So all I'm proposing is we
remove the other half.

I've not seen myself, nor can I find an example online where the
primary failed yet the secondary did not also fail from the same
cause.

If it is a possibility to do this, now we have pg_waldump we can
easily search for a different checkpoint and start from there instead
which is a more flexible approach. If you didn't save your WAL and
don't have any other form of backup, relying on the secondary
checkpoint is not exactly a safe bet.

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