Re: [patch] [doc] Further note required activity aspect of automatic checkpoint and archving

On Fri, Jan 15, 2021 at 12:16 AM Peter Eisentraut <
peter(dot)eisentraut(at)enterprisedb(dot)com> wrote:

> On 2020-10-12 23:54, David G. Johnston wrote:
> > --- a/doc/src/sgml/backup.sgml
> > +++ b/doc/src/sgml/backup.sgml
> > @@ -722,6 +722,8 @@ test ! -f
> > /mnt/server/archivedir/00000001000000A900000065 &amp;&amp; cp pg_wal/0
> > short <varname>archive_timeout</varname> &mdash; it will bloat
> > your archive
> > storage. <varname>archive_timeout</varname> settings of a minute
> > or so are
> > usually reasonable.
> > + This is mitigated by the fact that empty WAL segments will not be
> > archived
> > + even if the archive_timeout period has elapsed.
> > </para>
>
> This is hopefully not what happens. What this would mean is that I'd
> then have a sequence of WAL files named, say,
>
> 1, 2, 3, 7, 8, ...
>
> because a few in the middle were not archived because they were empty.
>

This addition assumes it is known that the archive process first fills the
files to their maximum size and then archives them. That filling of the
file is what causes the next file in the sequence to be created. So if the
archiving doesn't happen the files do not get filled and the status-quo
prevails.

If the above wants to be made more explicit in this change maybe:

"This is mitigated by the fact that archiving, and thus filling, the active
WAL segment will not happen if that segment is empty; it will continue as
the active segment."

> > --- a/doc/src/sgml/config.sgml
> > +++ b/doc/src/sgml/config.sgml
> > @@ -3131,6 +3131,8 @@ include_dir 'conf.d'
> > <listitem>
> > <para>
> > Maximum time between automatic WAL checkpoints.
> > + The automatic checkpoint will do nothing if no new WAL has been
> > + written since the last recorded checkpoint.
> > If this value is specified without units, it is taken as
> seconds.
> > The valid range is between 30 seconds and one day.
> > The default is five minutes (<literal>5min</literal>).
>
> I think what happens is that the checkpoint is skipped, not that the
> checkpoint happens but does nothing. That is the wording you cited in
> the other thread from
> <https://www.postgresql.org/docs/13/wal-configuration.html>.
>

Consistency is good; and considering it further the skipped wording is
generally better anyway.

"The automatic checkpoint will be skipped if no new WAL has been written
since the last recorded checkpoint."

David J.