Defer flushing of SLRU files.

Previously, we called fsync() after writing out individual pg_xact,
pg_multixact and pg_commit_ts pages due to cache pressure, leading to
regular I/O stalls in user backends and recovery.  Collapse requests for
the same file into a single system call as part of the next checkpoint,
as we already did for relation files, using the infrastructure developed
by commit 3eb77eba.  This can cause a significant improvement to
recovery performance, especially when it's otherwise CPU-bound.

Hoist ProcessSyncRequests() up into CheckPointGuts() to make it clearer
that it applies to all the SLRU mini-buffer-pools as well as the main
buffer pool.  Rearrange things so that data collected in CheckpointStats
includes SLRU activity.

Also remove the Shutdown{CLOG,CommitTS,SUBTRANS,MultiXact}() functions,
because they were redundant after the shutdown checkpoint that
immediately precedes them.  (I'm not sure if they were ever needed, but
they aren't now.)

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us> (parts)
Tested-by: Jakub Wartak <Jakub.Wartak@tomtom.com>
Discussion: https://postgr.es/m/CA+hUKGLJ=84YT+NvhkEEDAuUtVHMfQ9i-N7k_o50JmQ6Rpj_OQ@mail.gmail.com

1 files changed, 18 insertions, 10 deletions

diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index 61754312e26..79a77ebbfe2 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -8528,10 +8528,6 @@ ShutdownXLOG(int code, Datum arg)
 
 		CreateCheckPoint(CHECKPOINT_IS_SHUTDOWN | CHECKPOINT_IMMEDIATE);
 	}
-	ShutdownCLOG();
-	ShutdownCommitTs();
-	ShutdownSUBTRANS();
-	ShutdownMultiXact();
 }
 
 /*
@@ -9176,17 +9172,29 @@ CreateEndOfRecoveryRecord(void)
 static void
 CheckPointGuts(XLogRecPtr checkPointRedo, int flags)
 {
-	CheckPointCLOG();
-	CheckPointCommitTs();
-	CheckPointSUBTRANS();
-	CheckPointMultiXact();
-	CheckPointPredicate();
 	CheckPointRelationMap();
 	CheckPointReplicationSlots();
 	CheckPointSnapBuild();
 	CheckPointLogicalRewriteHeap();
-	CheckPointBuffers(flags);	/* performs all required fsyncs */
 	CheckPointReplicationOrigin();
+
+	/* Write out all dirty data in SLRUs and the main buffer pool */
+	TRACE_POSTGRESQL_BUFFER_CHECKPOINT_START(flags);
+	CheckpointStats.ckpt_write_t = GetCurrentTimestamp();
+	CheckPointCLOG();
+	CheckPointCommitTs();
+	CheckPointSUBTRANS();
+	CheckPointMultiXact();
+	CheckPointPredicate();
+	CheckPointBuffers(flags);
+
+	/* Perform all queued up fsyncs */
+	TRACE_POSTGRESQL_BUFFER_CHECKPOINT_SYNC_START();
+	CheckpointStats.ckpt_sync_t = GetCurrentTimestamp();
+	ProcessSyncRequests();
+	CheckpointStats.ckpt_sync_end_t = GetCurrentTimestamp();
+	TRACE_POSTGRESQL_BUFFER_CHECKPOINT_DONE();
+
 	/* We deliberately delay 2PC checkpointing as long as possible */
 	CheckPointTwoPhase(checkPointRedo);
 }