pgstat: Infrastructure for more detailed IO statistics

This commit adds the infrastructure for more detailed IO statistics. The calls
to actually count IOs, a system view to access the new statistics,
documentation and tests will be added in subsequent commits, to make review
easier.

While we already had some IO statistics, e.g. in pg_stat_bgwriter and
pg_stat_database, they did not provide sufficient detail to understand what
the main sources of IO are, or whether configuration changes could avoid
IO. E.g., pg_stat_bgwriter.buffers_backend does contain the number of buffers
written out by a backend, but as that includes extending relations (always
done by backends) and writes triggered by the use of buffer access strategies,
it cannot easily be used to tune background writer or checkpointer. Similarly,
pg_stat_database.blks_read cannot easily be used to tune shared_buffers /
compute a cache hit ratio, as the use of buffer access strategies will often
prevent a large fraction of the read blocks to end up in shared_buffers.

The new IO statistics count IO operations (evict, extend, fsync, read, reuse,
and write), and are aggregated for each combination of backend type (backend,
autovacuum worker, bgwriter, etc), target object of the IO (relations, temp
relations) and context of the IO (normal, vacuum, bulkread, bulkwrite).

What is tracked in this series of patches, is sufficient to perform the
aforementioned analyses. Further details, e.g. tracking the number of buffer
hits, would make that even easier, but was left out for now, to keep the scope
of the already large patchset manageable.

Bumps PGSTAT_FILE_FORMAT_ID.

Author: Melanie Plageman <melanieplageman@gmail.com>
Reviewed-by: Andres Freund <andres@anarazel.de>
Reviewed-by: Justin Pryzby <pryzby@telsasoft.com>
Reviewed-by: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Discussion: https://postgr.es/m/20200124195226.lth52iydq2n2uilq@alap3.anarazel.de

1 files changed, 13 insertions, 2 deletions

diff --git a/src/backend/utils/activity/pgstat_relation.c b/src/backend/utils/activity/pgstat_relation.c
index 2e20b93c202..f793ac15165 100644
--- a/src/backend/utils/activity/pgstat_relation.c
+++ b/src/backend/utils/activity/pgstat_relation.c
@@ -206,7 +206,7 @@ pgstat_drop_relation(Relation rel)
 }
 
 /*
- * Report that the table was just vacuumed.
+ * Report that the table was just vacuumed and flush IO statistics.
  */
 void
 pgstat_report_vacuum(Oid tableoid, bool shared,
@@ -258,10 +258,18 @@ pgstat_report_vacuum(Oid tableoid, bool shared,
 	}
 
 	pgstat_unlock_entry(entry_ref);
+
+	/*
+	 * Flush IO statistics now. pgstat_report_stat() will flush IO stats,
+	 * however this will not be called until after an entire autovacuum cycle
+	 * is done -- which will likely vacuum many relations -- or until the
+	 * VACUUM command has processed all tables and committed.
+	 */
+	pgstat_flush_io(false);
 }
 
 /*
- * Report that the table was just analyzed.
+ * Report that the table was just analyzed and flush IO statistics.
  *
  * Caller must provide new live- and dead-tuples estimates, as well as a
  * flag indicating whether to reset the mod_since_analyze counter.
@@ -341,6 +349,9 @@ pgstat_report_analyze(Relation rel,
 	}
 
 	pgstat_unlock_entry(entry_ref);
+
+	/* see pgstat_report_vacuum() */
+	pgstat_flush_io(false);
 }
 
 /*