Re: Speed up pg_checksums in cases where checksum already set

On Thu, May 27, 2021 at 10:29:14AM -0400, Greg Sabino Mullane wrote:
> I was originally on the fence about including this as well, but it seems
> like since the database is shut down and already in a consistent state,
> there seems no advantage to syncing if we have not made any changes. Things
> are no better or worse than when we arrived. However, the real-world use
> case of running pg_checksums --enable and getting no changed blocks is
> probably fairly rare, so if there is a strong objection, I'm happy
> reverting to just (do_sync). (I'm not sure how cheap a sync is, I assume
> it's low impact as the database is shut down, I guess it becomes a "might
> as well while we are here"?)

I understand that this should be rare, but I don't want to take any
bets either.  With this patch, we could finish with cases where some
pages are still in the OS cache but don't get flushed because a
previous cancellation let the cluster in a state where all the page
checksums have been written out but a portion of the files were not
synced.  A follow-up run of pg_checksums would see that all the pages
are correct, but would think that no sync is required, incorrectly.
--
Michael