doc: Fix obsolete description about pg_basebackup.

MasaoFujii · MasaoFujii · commit eaec48b3c54e · 2021-04-23T15:45:46.000+09:00
Previously it was documented that if using "-X none" option there was no guarantee that all required WAL files were archived at the end of pg_basebackup when taking a backup from the standby. But this limitation was removed by commit 52f8a59. Now, even when taking a backup from the standby, pg_basebackup can wait for all required WAL files to be archived. Therefore this commit removes such obsolete description from the docs. Also this commit adds new description about the limitation when taking a backup from the standby, into the docs. The limitation is that pg_basebackup cannot force the standbfy to switch to a new WAL file at the end of backup, which may cause pg_basebackup to wait a long time for the last required WAL file to be switched and archived, especially when write activity on the primary is low. Back-patch to v10 where the issue was introduced. Reported-by: Kyotaro Horiguchi Author: Kyotaro Horiguchi, Fujii Masao Reviewed-by: Kyotaro Horiguchi, Fujii Masao Discussion: https://postgr.es/m/20210420.133235.1342729068750553399.horikyota.ntt@gmail.com
diff --git a/doc/src/sgml/ref/pg_basebackup.sgml b/doc/src/sgml/ref/pg_basebackup.sgml
@@ -83,8 +83,14 @@ PostgreSQL documentation
     </listitem>
     <listitem>
      <para>
-      If you are using <literal>-X none</literal>, there is no guarantee that all
-      WAL files required for the backup are archived at the end of backup.
+      <application>pg_basebackup</application> cannot force the standby
+      to switch to a new WAL file at the end of backup.
+      When you are using <literal>-X none</literal>, if write activity on
+      the primary is low, <application>pg_basebackup</application> may
+      need to wait a long time for the last WAL file required for the backup
+      to be switched and archived.  In this case, it may be useful to run
+      <function>pg_switch_wal</function> on the primary in order to
+      trigger an immediate WAL file switch.
      </para>
     </listitem>
     <listitem>
