Optionally prefetch referenced data in recovery.

Introduce a new GUC recovery_prefetch, disabled by default.  When
enabled, look ahead in the WAL and try to initiate asynchronous reading
of referenced data blocks that are not yet cached in our buffer pool.
For now, this is done with posix_fadvise(), which has several caveats.
Better mechanisms will follow in later work on the I/O subsystem.

The GUC maintenance_io_concurrency is used to limit the number of
concurrent I/Os we allow ourselves to initiate, based on pessimistic
heuristics used to infer that I/Os have begun and completed.

The GUC wal_decode_buffer_size is used to limit the maximum distance we
are prepared to read ahead in the WAL to find uncached blocks.

Reviewed-by: Alvaro Herrera <alvherre@2ndquadrant.com> (parts)
Reviewed-by: Andres Freund <andres@anarazel.de> (parts)
Reviewed-by: Tomas Vondra <tomas.vondra@2ndquadrant.com> (parts)
Tested-by: Tomas Vondra <tomas.vondra@2ndquadrant.com>
Tested-by: Jakub Wartak <Jakub.Wartak@tomtom.com>
Tested-by: Dmitry Dolgov <9erthalion6@gmail.com>
Tested-by: Sait Talha Nisanci <Sait.Nisanci@microsoft.com>
Discussion: https://postgr.es/m/CA%2BhUKGJ4VJN8ttxScUFM8dOKX0BrBiboo5uz1cq%3DAovOddfHpA%40mail.gmail.com

1 files changed, 14 insertions, 0 deletions

diff --git a/src/backend/catalog/system_views.sql b/src/backend/catalog/system_views.sql
index ff65b3edfa..451db2ee0a 100644
--- a/src/backend/catalog/system_views.sql
+++ b/src/backend/catalog/system_views.sql
@@ -911,6 +911,20 @@ CREATE VIEW pg_stat_wal_receiver AS
     FROM pg_stat_get_wal_receiver() s
     WHERE s.pid IS NOT NULL;
 
+CREATE VIEW pg_stat_prefetch_recovery AS
+    SELECT
+            s.stats_reset,
+            s.prefetch,
+            s.skip_hit,
+            s.skip_new,
+            s.skip_fpw,
+            s.skip_seq,
+            s.distance,
+            s.queue_depth,
+	    s.avg_distance,
+	    s.avg_queue_depth
+     FROM pg_stat_get_prefetch_recovery() s;
+
 CREATE VIEW pg_stat_subscription AS
     SELECT
             su.oid AS subid,