On Wed, May 26, 2021 at 09:29:43PM -0500, Justin Pryzby wrote: > In one of the checksum patches, there was an understanding that the pages > should be written even if the checksum is correct, to handle replicas. > > From the v19 patch: > https://www.postgresql.org/message-id/F7AFCFCD-8F77-4546-8D42-C7F675A4B680%40yesql.se > + * Mark the buffer as dirty and force a full page write. We > have to > + * re-write the page to WAL even if the checksum hasn't > changed, > + * because if there is a replica it might have a slightly > different > + * version of the page with an invalid checksum, caused by > unlogged > + * changes (e.g. hintbits) on the master happening while > checksums > + * were off. This can happen if there was a valid checksum on > the page > + * at one point in the past, so only when checksums are first > on, then > + * off, and then turned on again.
I am not really following the line of argument here. pg_checksums relies on the fact that the cluster has been safely shut down before running. So, if this comes to standbys, they would have reached a consistent point, and the shutdown makes sure that all pages are flushed. -- Michael
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