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 jumping forward 32KB or less.
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.

Author: jimis

src/bin/pg_dump/pg_backup_custom.c

@@ -629,7 +629,11 @@ _skipData(ArchiveHandle *AH)
 	blkLen = ReadInt(AH);
 	while (blkLen != 0)
 	{
-		if (ctx->hasSeek)
+		/*
+		 * Sequential access is usually faster, so avoid seeking if the jump
+		 * forward is 32KB or less.
+		 */
+		if (ctx->hasSeek && blkLen > 32 * 1024)
 		{
 			if (fseeko(AH->FH, blkLen, SEEK_CUR) != 0)
 				pg_fatal("error during file seek: %m");