Re: Online enabling of checksums

On Fri, Mar 2, 2018 at 8:35 AM, Magnus Hagander <magnus(at)hagander(dot)net> wrote:
> Do we ever make hintbit changes on the standby for example? If so, it would
> definitely cause problems. I didn't realize we did, actually...

We do not.

> I guess we could get there even if we don't by:
> * All checksums are correct
> * Checkums are disabled (which replicates)
> * Non-WAL logged change on the master, which updates checksum but does *not*
> replicate
> * Checksums re-enabled
> * Worker sees the checksum as correct, and thus does not force a full page
> write.
> * Worker completes and flips checksums on which replicates. At this point,
> if the replica reads the page, boom.

Exactly.

> I guess we have to remove that optimisation. It's definitely a bummer, but I
> don't think it's an absolute dealbreaker.

I don't disagree.

> We could say that we keep the optimisation if wal_level=minimal for example,
> because then we know there is no replica. But I doubt that's worth it?

I don't have a strong feeling about this.

--
Robert Haas
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