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