v4-0001-parallel-pg_restore-avoid-disk-seeks-when-moving-.patch

From dcbbe92ba16b0c1dfe6320960bc2882b15850de6 Mon Sep 17 00:00:00 2001
From: Dimitrios Apostolou <jimis@qt.io>
Date: Sat, 29 Mar 2025 01:16:07 +0100
Subject: [PATCH v4] parallel pg_restore: avoid disk seeks when moving short
 distance forward

Improve the performance of parallel pg_restore (-j) from a custom format
pg_dump archive that does not include data offsets - typically happening
when pg_dump has generated it by writing to stdout instead of a file.
Also speeds up restoration of specific tables (-t tablename).

In these cases, before the actual data restoration starts, pg_restore
workers manifest constant looping of reading small sizes (4KB) and
seeking forward small lenths (around 10KB for a compressed archive or
even only a few bytes for uncompressed ones):

read(4, "..."..., 4096) = 4096
lseek(4, 55544369152, SEEK_SET)         = 55544369152
read(4, "..."..., 4096) = 4096
lseek(4, 55544381440, SEEK_SET)         = 55544381440
read(4, "..."..., 4096) = 4096
lseek(4, 55544397824, SEEK_SET)         = 55544397824
read(4, "..."..., 4096) = 4096
lseek(4, 55544414208, SEEK_SET)         = 55544414208
read(4, "..."..., 4096) = 4096
lseek(4, 55544426496, SEEK_SET)         = 55544426496

This happens as each worker has to scan the whole file until it finds
the entry it wants, skipping forward each block. In combination to the
small block size of the custom format dump, this causes many seeks and
low performance.

Fix by avoiding forward seeks for jumps of less than 1MB forward.
Do instead sequential reads.

Performance gain can be significant, depending on the size of the dump
and the I/O subsystem. On my local NVMe drive, read speeds for that
phase of pg_restore increased from 150MB/s to 3GB/s.
---
 src/bin/pg_dump/pg_backup_custom.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/src/bin/pg_dump/pg_backup_custom.c b/src/bin/pg_dump/pg_backup_custom.c
index f7c3af56304..27695e24dde 100644
--- a/src/bin/pg_dump/pg_backup_custom.c
+++ b/src/bin/pg_dump/pg_backup_custom.c
@@ -623,19 +623,23 @@ _skipData(ArchiveHandle *AH)
 {
 	lclContext *ctx = (lclContext *) AH->formatData;
 	size_t		blkLen;
 	char	   *buf = NULL;
 	int			buflen = 0;
 
 	blkLen = ReadInt(AH);
 	while (blkLen != 0)
 	{
-		if (ctx->hasSeek)
+		/*
+		 * Sequential access is usually faster, so avoid seeking if the jump
+		 * forward is shorter than 1MB.
+		 */
+		if (ctx->hasSeek && blkLen > 1024 * 1024)
 		{
 			if (fseeko(AH->FH, blkLen, SEEK_CUR) != 0)
 				pg_fatal("error during file seek: %m");
 		}
 		else
 		{
 			if (blkLen > buflen)
 			{
 				free(buf);
-- 
2.51.0