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

At Fri, 24 Apr 2020 16:29:41 +0900, Michael Paquier <michael@paquier.xyz> wrote in 
> 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?

Oops, I'm the one to blame on it. Actually it is a mistake of
XLOG_SWITCH.  By the way, the "xlog-switch" in the following comment
might be better to be XLOG_SWITCH.

>    /* All (non xlog-switch) records should contain data. */

There are several other occurances of xlog-switch.

ragards.

-- 
Kyotaro Horiguchi
NTT Open Source Software Center