RE: About to add WAL write/fsync statistics to pg_stat_wal view

Dear Ikeda-san,

Thank you for updating the patch. This can be applied to master, and
can be used on my RHEL7.
wal_write_time and wal_sync_time increase normally :-).

```
postgres=# select * from pg_stat_wal;
-[ RECORD 1 ]----+------------------------------
wal_records      | 121781
wal_fpi          | 2287
wal_bytes        | 36055146
wal_buffers_full | 799
wal_write        | 12770
wal_write_time   | 4.469
wal_sync         | 11962
wal_sync_time    | 132.352
stats_reset      | 2021-01-25 00:51:40.674412+00
```

I put a further comment:

```
@@ -3485,7 +3485,53 @@ SELECT pid, wait_event_type, wait_event FROM pg_stat_activity WHERE wait_event i
        <structfield>wal_buffers_full</structfield> <type>bigint</type>
       </para>
       <para>
-       Number of times WAL data was written to disk because WAL buffers became full
+       Total number of times WAL data was written to disk because WAL buffers became full
+      </para></entry>
+     </row>
+
+     <row>
+      <entry role="catalog_table_entry"><para role="column_definition">
+       <structfield>wal_write</structfield> <type>bigint</type>
+      </para>
+      <para>
+       Total number of times WAL data was written to disk
+      </para></entry>
+     </row>
+
+     <row>
+      <entry role="catalog_table_entry"><para role="column_definition">
+       <structfield>wal_write_time</structfield> <type>double precision</type>
+      </para>
+      <para>
+       Total amount of time that has been spent in the portion of
+       WAL data was written to disk, in milliseconds
+       (if <xref linkend="guc-track-wal-io-timing"/> is enabled, otherwise zero).
+      </para></entry>
+     </row>
+
+     <row>
+      <entry role="catalog_table_entry"><para role="column_definition">
+       <structfield>wal_sync</structfield> <type>bigint</type>
+      </para>
+      <para>
+       Total number of times WAL data was synced to disk
+       (if <xref linkend="guc-wal-sync-method"/> is <literal>open_datasync</literal> or 
+       <literal>open_sync</literal>, this value is zero because WAL data is synced 
+       when to write it).
+      </para></entry>
+     </row>
+
+     <row>
+      <entry role="catalog_table_entry"><para role="column_definition">
+       <structfield>wal_sync_time</structfield> <type>double precision</type>
+      </para>
+      <para>
+       Total amount of time that has been spent in the portion of
+       WAL data was synced to disk, in milliseconds
+       (if <xref linkend="guc-track-wal-io-timing"/> is enabled, otherwise zero.
+       if <xref linkend="guc-wal-sync-method"/> is <literal>open_datasync</literal> or 
+       <literal>open_sync</literal>, this value is zero too because WAL data is synced 
+       when to write it).
       </para></entry>
      </row>
 ```

Maybe "Total amount of time" should be used, not "Total number of time."
Other views use "amount."

I have no comments anymore.

Hayato Kuroda
FUJITSU LIMITED