Rename "pg_xlog" directory to "pg_wal".

"xlog" is not a particularly clear abbreviation for "write-ahead log",
and it sometimes confuses users into believe that the contents of the
"pg_xlog" directory are not critical data, leading to unpleasant
consequences.  So, rename the directory to "pg_wal".

This patch modifies pg_upgrade and pg_basebackup to understand both
the old and new directory layouts; the former is necessary given the
purpose of the tool, while the latter merely avoids an unnecessary
backward-compatibility break.

We may wish to consider renaming other programs, switches, and
functions which still use the old "xlog" naming to also refer to
"wal".  However, that's still under discussion, so let's do just this
much for now.

Discussion: CAB7nPqTeC-8+zux8_-4ZD46V7YPwooeFxgndfsq5Rg8ibLVm1A@mail.gmail.com

Michael Paquier

doc/src/sgml/ref/pgupgrade.sgml

@@ -345,7 +345,7 @@ NET STOP postgresql-9.0
      your old cluster
      once you start the new cluster after the upgrade.  Link mode also
      requires that the old and new cluster data directories be in the
-     same file system.  (Tablespaces and <filename>pg_xlog</> can be on
+     same file system.  (Tablespaces and <filename>pg_wal</> can be on
      different file systems.)  See <literal>pg_upgrade --help</> for a full
      list of options.
     </para>
@@ -508,7 +508,7 @@ rsync --archive --delete --hard-links --size-only old_pgdata new_pgdata remote_d
       <para>
        If you have tablespaces, you will need to run a similar
        <application>rsync</> command for each tablespace directory.  If you
-       have relocated <filename>pg_xlog</> outside the data directories,
+       have relocated <filename>pg_wal</> outside the data directories,
        <application>rsync</> must be run on those directories too.
       </para>
      </step>