*** a/src/backend/access/transam/xlog.c
--- b/src/backend/access/transam/xlog.c
***************
*** 9311,9316 **** do_pg_start_backup(const char *backupidstr, bool fast, char **labelfile)
--- 9311,9317 ----
  								BACKUP_LABEL_FILE)));
  			if (fwrite(labelfbuf.data, labelfbuf.len, 1, fp) != 1 ||
  				fflush(fp) != 0 ||
+ 				pg_fsync(fileno(fp)) != 0 ||
  				ferror(fp) ||
  				FreeFile(fp))
  				ereport(ERROR,