doc: Add details about object "wal" in pg_stat_io

This commit adds a short description of what kind of activity is tracked
in pg_stat_io for the object "wal", with a link pointing to the section
"WAL configuration" that has a lot of details on the matter.

This should perhaps have been added in a051e71e28a1, but things are what
they are.

Author: Bertrand Drouvot 
Discussion: https://postgr.es/m/Z7RkQ0EfYaqqjgz/@ip-10-97-1-34.eu-west-3.compute.internal

diff --git a/doc/src/sgml/monitoring.sgml b/doc/src/sgml/monitoring.sgml
index 928a6eb64b07599ed549ce41062cb40e562eb359..71c4f96d0544bb017d8607e85db8dc5d5b3dfac0 100644 (file)
--- a/doc/src/sgml/monitoring.sgml
+++ b/doc/src/sgml/monitoring.sgml
@@ -2930,6 +2930,16 @@ description | Waiting for a newly initialized WAL file to reach durable storage
    writer.
   
 
+  
+   For the object wal,
+   fsyncs and fsync_time track the
+   fsync activity of WAL files done in issue_xlog_fsync.
+   writes and write_time
+   track the write activity of WAL files done in
+   XLogWrite.
+   See  for more information.
+  
+
   
    pg_stat_io can be used to inform database tuning.
    For example: