From 0d9bd67772dbcec8b04dcd2c45abb4c1fa1c2b86 Mon Sep 17 00:00:00 2001
From: Nazir Bilal Yavuz <byavuz81@gmail.com>
Date: Tue, 16 Jul 2024 18:11:21 +0300
Subject: [PATCH v4 3/3] Use read streams in CREATE DATABASE when strategy is
 wal_log

CREATE DABASE command uses RelationCopyStorageUsingBuffer() function to
copy buffers when the strategy is wal_log. This function reads source
buffers then copies them to the destination buffers. Read streams are
used only only while reading source buffers because the destination
buffer is read by 'RBM_ZERO_AND_LOCK' option, so it is not important.
---
 src/backend/storage/buffer/bufmgr.c | 53 ++++++++++++++++++++++++++---
 1 file changed, 49 insertions(+), 4 deletions(-)

diff --git a/src/backend/storage/buffer/bufmgr.c b/src/backend/storage/buffer/bufmgr.c
index 61e8f726268..24a8ea6611b 100644
--- a/src/backend/storage/buffer/bufmgr.c
+++ b/src/backend/storage/buffer/bufmgr.c
@@ -54,6 +54,7 @@
 #include "storage/ipc.h"
 #include "storage/lmgr.h"
 #include "storage/proc.h"
+#include "storage/read_stream.h"
 #include "storage/smgr.h"
 #include "storage/standby.h"
 #include "utils/memdebug.h"
@@ -135,6 +136,33 @@ typedef struct SMgrSortArray
 	SMgrRelation srel;
 } SMgrSortArray;
 
+/*
+ * Helper struct for read stream object used in
+ * RelationCopyStorageUsingBuffer() function.
+ */
+struct copy_storage_using_buffer_read_stream_private
+{
+	BlockNumber blocknum;
+	BlockNumber last_block;
+};
+
+/*
+ * Callback function to get next block for read stream object used in
+ * RelationCopyStorageUsingBuffer() function.
+ */
+static BlockNumber
+copy_storage_using_buffer_read_stream_next_block(ReadStream *stream,
+												 void *callback_private_data,
+												 void *per_buffer_data)
+{
+	struct copy_storage_using_buffer_read_stream_private *p = callback_private_data;
+
+	if (p->blocknum < p->last_block)
+		return p->blocknum++;
+
+	return InvalidBlockNumber;
+}
+
 /* GUC variables */
 bool		zero_damaged_pages = false;
 int			bgwriter_lru_maxpages = 100;
@@ -4687,6 +4715,9 @@ RelationCopyStorageUsingBuffer(RelFileLocator srclocator,
 	PGIOAlignedBlock buf;
 	BufferAccessStrategy bstrategy_src;
 	BufferAccessStrategy bstrategy_dst;
+	struct copy_storage_using_buffer_read_stream_private p;
+	ReadStream *src_stream;
+	SMgrRelation src_smgr;
 
 	/*
 	 * In general, we want to write WAL whenever wal_level > 'minimal', but we
@@ -4715,19 +4746,31 @@ RelationCopyStorageUsingBuffer(RelFileLocator srclocator,
 	bstrategy_src = GetAccessStrategy(BAS_BULKREAD);
 	bstrategy_dst = GetAccessStrategy(BAS_BULKWRITE);
 
+	/* Initalize streaming read */
+	p.blocknum = 0;
+	p.last_block = nblocks;
+	src_smgr = smgropen(srclocator, INVALID_PROC_NUMBER);
+	src_stream = read_stream_begin_smgr_relation(READ_STREAM_FULL,
+												 bstrategy_src,
+												 src_smgr,
+												 permanent ? RELPERSISTENCE_PERMANENT : RELPERSISTENCE_UNLOGGED,
+												 forkNum,
+												 copy_storage_using_buffer_read_stream_next_block,
+												 &p,
+												 0);
+
 	/* Iterate over each block of the source relation file. */
 	for (blkno = 0; blkno < nblocks; blkno++)
 	{
 		CHECK_FOR_INTERRUPTS();
 
 		/* Read block from source relation. */
-		srcBuf = ReadBufferWithoutRelcache(srclocator, forkNum, blkno,
-										   RBM_NORMAL, bstrategy_src,
-										   permanent);
+		srcBuf = read_stream_next_buffer(src_stream, NULL);
 		LockBuffer(srcBuf, BUFFER_LOCK_SHARE);
 		srcPage = BufferGetPage(srcBuf);
 
-		dstBuf = ReadBufferWithoutRelcache(dstlocator, forkNum, blkno,
+		dstBuf = ReadBufferWithoutRelcache(dstlocator, forkNum,
+										   BufferGetBlockNumber(srcBuf),
 										   RBM_ZERO_AND_LOCK, bstrategy_dst,
 										   permanent);
 		dstPage = BufferGetPage(dstBuf);
@@ -4747,6 +4790,8 @@ RelationCopyStorageUsingBuffer(RelFileLocator srclocator,
 		UnlockReleaseBuffer(dstBuf);
 		UnlockReleaseBuffer(srcBuf);
 	}
+	Assert(read_stream_next_buffer(src_stream, NULL) == InvalidBuffer);
+	read_stream_end(src_stream);
 
 	FreeAccessStrategy(bstrategy_src);
 	FreeAccessStrategy(bstrategy_dst);
-- 
2.45.2