Re: pgsql: Skip WAL for new relfilenodes, under wal_level=minimal.

Hi Noah,

On Sat, Apr 04, 2020 at 08:40:43PM +0000, Noah Misch wrote:
> Skip WAL for new relfilenodes, under wal_level=minimal.
>
> Until now, only selected bulk operations (e.g. COPY) did this.  If a
> given relfilenode received both a WAL-skipping COPY and a WAL-logged
> operation (e.g. INSERT), recovery could lose tuples from the COPY.  See
> src/backend/access/transam/README section "Skipping WAL for New
> RelFileNode" for the new coding rules.  Maintainers of table access
> methods should examine that section.
>
> To maintain data durability, just before commit, we choose between an
> fsync of the relfilenode and copying its contents to WAL.  A new GUC,
> wal_skip_threshold, guides that choice.  If this change slows a workload
> that creates small, permanent relfilenodes under wal_level=minimal, try
> adjusting wal_skip_threshold.  Users setting a timeout on COMMIT may
> need to adjust that timeout, and log_min_duration_statement analysis
> will reflect time consumption moving to COMMIT from commands like COPY.

+   /*
+    * Records other than SWITCH_WAL must have content. We use an integer 0 to
+    * follow the restriction.
+    */
This commit has added the following comment, but I guess you meant
XLOG_SWITCH instead?
--
Michael