Use the buffer cache when initializing an unlogged index.

Some of the ambuildempty functions used smgrwrite() directly, followed
by smgrimmedsync(). A few small problems with that:

Firstly, one is supposed to use smgrextend() when extending a
relation, not smgrwrite(). It doesn't make much difference in
production builds. smgrextend() updates the relation size cache, so
you miss that, but that's harmless because we never use the cached
relation size of an init fork. But if you compile with
CHECK_WRITE_VS_EXTEND, you get an assertion failure.

Secondly, the smgrwrite() calls were performed before WAL-logging, so
the page image written to disk had 0/0 as the LSN, not the LSN of the
WAL record. That's also harmless in practice, but seems sloppy.

Thirdly, it's better to use the buffer cache, because then you don't
need to smgrimmedsync() the relation to disk, which adds latency.
Bypassing the cache makes sense for bulk operations like index
creation, but not when you're just initializing an empty index.
Creation of unlogged tables is hardly performance bottleneck in any
real world applications, but nevertheless.

Backpatch to v16, but no further. These issues should be harmless in
practice, so better to not rock the boat in older branches.

Reviewed-by: Robert Haas
Discussion: https://www.postgresql.org/message-id/6e5bbc08-cdfc-b2b3-9e23-1a914b9850a9@iki.fi

1 files changed, 21 insertions, 20 deletions

diff --git a/src/backend/access/nbtree/nbtree.c b/src/backend/access/nbtree/nbtree.c
index 4553aaee531..ad07b80f758 100644
--- a/src/backend/access/nbtree/nbtree.c
+++ b/src/backend/access/nbtree/nbtree.c
@@ -151,31 +151,32 @@ bthandler(PG_FUNCTION_ARGS)
 void
 btbuildempty(Relation index)
 {
+	Buffer		metabuf;
 	Page		metapage;
 
-	/* Construct metapage. */
-	metapage = (Page) palloc_aligned(BLCKSZ, PG_IO_ALIGN_SIZE, 0);
-	_bt_initmetapage(metapage, P_NONE, 0, _bt_allequalimage(index, false));
-
 	/*
-	 * Write the page and log it.  It might seem that an immediate sync would
-	 * be sufficient to guarantee that the file exists on disk, but recovery
-	 * itself might remove it while replaying, for example, an
-	 * XLOG_DBASE_CREATE* or XLOG_TBLSPC_CREATE record.  Therefore, we need
-	 * this even when wal_level=minimal.
+	 * Initalize the metapage.
+	 *
+	 * Regular index build bypasses the buffer manager and uses smgr functions
+	 * directly, with an smgrimmedsync() call at the end.  That makes sense
+	 * when the index is large, but for an empty index, it's better to use the
+	 * buffer cache to avoid the smgrimmedsync().
 	 */
-	PageSetChecksumInplace(metapage, BTREE_METAPAGE);
-	smgrwrite(RelationGetSmgr(index), INIT_FORKNUM, BTREE_METAPAGE,
-			  metapage, true);
-	log_newpage(&RelationGetSmgr(index)->smgr_rlocator.locator, INIT_FORKNUM,
-				BTREE_METAPAGE, metapage, true);
+	metabuf = ReadBufferExtended(index, INIT_FORKNUM, P_NEW, RBM_NORMAL, NULL);
+	Assert(BufferGetBlockNumber(metabuf) == BTREE_METAPAGE);
+	_bt_lockbuf(index, metabuf, BT_WRITE);
 
-	/*
-	 * An immediate sync is required even if we xlog'd the page, because the
-	 * write did not go through shared_buffers and therefore a concurrent
-	 * checkpoint may have moved the redo pointer past our xlog record.
-	 */
-	smgrimmedsync(RelationGetSmgr(index), INIT_FORKNUM);
+	START_CRIT_SECTION();
+
+	metapage = BufferGetPage(metabuf);
+	_bt_initmetapage(metapage, P_NONE, 0, _bt_allequalimage(index, false));
+	MarkBufferDirty(metabuf);
+	log_newpage_buffer(metabuf, true);
+
+	END_CRIT_SECTION();
+
+	_bt_unlockbuf(index, metabuf);
+	ReleaseBuffer(metabuf);
 }
 
 /*