pg_rewind: Skip copy of WAL segments generated before point of divergence

This commit makes the way WAL segments are handled from the source to
the target server slightly smarter: the copy of the WAL segments is now
skipped if these have been created before the point where source and
target have diverged (the WAL segment where the point of divergence
exists is still copied), because we know that such segments exist on
both the target and source.  Note that the on-disk size of the WAL
segments on the source and target need to match.  Hence, only the
segments generated after the point of divergence are now copied.  A
segment existing on the source but not the target is copied.

Previously, all the WAL segments were just copied in full.  This change
can make the rewind operation cheaper in some configurations, especially
for setups where some WAL retention causes many segments to remain on
the source server even after the promotion of a standby used as source
to rewind a previous primary.

A TAP test is added to track these new behaviors.  The file map printed
with --debug now includes all the information related to WAL segments,
to be able to track if these are copied or skipped, and the test relies
on the debug output generated.

Author: John Hsu <johnhyvr@gmail.com>
Author: Justin Kwan <justinpkwan@outlook.com>
Reviewed-by: Robert Haas <robertmhaas@gmail.com>
Reviewed-by: Alexander Korotkov <aekorotkov@gmail.com>
Reviewed-by: Japin Li <japinli@hotmail.com>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Reviewed-by: Srinath Reddy Sadipiralla <srinath2133@gmail.com>
Discussion: https://postgr.es/m/181b4c6fa9c.b8b725681941212.7547232617810891479@viggy28.dev

1 files changed, 26 insertions, 6 deletions

diff --git a/doc/src/sgml/ref/pg_rewind.sgml b/doc/src/sgml/ref/pg_rewind.sgml
index 5485033ed8c..928e78cda33 100644
--- a/doc/src/sgml/ref/pg_rewind.sgml
+++ b/doc/src/sgml/ref/pg_rewind.sgml
@@ -52,12 +52,32 @@ PostgreSQL documentation
    analogous to a base backup of the source data directory. Unlike taking
    a new base backup or using a tool like <application>rsync</application>,
    <application>pg_rewind</application> does not require comparing or copying
-   unchanged relation blocks in the cluster. Only changed blocks from existing
-   relation files are copied; all other files, including new relation files,
-   configuration files, and WAL segments, are copied in full. As such the
-   rewind operation is significantly faster than other approaches when the
-   database is large and only a small fraction of blocks differ between the
-   clusters.
+   unchanged relation blocks in the cluster:
+  </para>
+  <itemizedlist>
+   <listitem>
+    <para>
+     Only changed blocks from existing relation files are copied.
+    </para>
+   </listitem>
+   <listitem>
+    <para>
+     WAL segments prior to the point where the source and target servers
+     have diverged are not copied. WAL segments generated after the source
+     and target servers have diverged are copied in full.
+    </para>
+   </listitem>
+   <listitem>
+    <para>
+     All other files, including new relation files and configuration files,
+     are copied in full.
+    </para>
+   </listitem>
+  </itemizedlist>
+  <para>
+   As such, the rewind operation is significantly faster than other
+   approaches when the database is large and only a small fraction of blocks
+   differ between the clusters.
   </para>
 
   <para>