Re: Running from 9.6 backups sometimes fails with fatal error

I started trying to debug backup with minimal WALs for achieve consistency, it is from 'server 2'
and it only needs 20 WAL files.

It error was:
ERROR: xlog flush request 44B/7E5DAB28 is not satisfied --- flushed only to 44B/7305B560
CONTEXT: writing block 0 of relation base/16506/16891_vm

=== backup_label ===
START WAL LOCATION: 44B/6002F280 (file 000000010000044B00000060)
CHECKPOINT LOCATION: 44B/7305B4F0
BACKUP METHOD: streamed
BACKUP FROM: standby
START TIME: 2019-02-13 03:19:54 MSK
LABEL: pg_basebackup base backup

=== controldata ===
Database cluster state: in archive recovery
pg_control last modified: Wed 13 Feb 2019 03:19:54 AM MSK
Latest checkpoint location: 44B/7305B4F0
Latest checkpoint's REDO location: 44B/6002F280
Latest checkpoint's REDO WAL file: 000000010000044B00000060
Time of latest checkpoint: Wed 13 Feb 2019 02:19:36 AM MSK
Minimum recovery ending location: 44B/7305B560

I scan all *_vm files in DB 16506 for pages with LSN bigger than 'Minimum recovery ending
location' and find three files:
base/16506/190024_vm 0000000 44B/9411F5C0
base/16506/190005_vm 0000000 44B/94121DE8
base/16506/16891_vm 0000000 44B/7E5DAB28

Then I parse (pg_xlogdump) all this 20 WALs, needed for consistency and search for this three
oid, for 190024 and 190005 I found VISIBLE WAL record:
=== $ grep 190024 wals-text | grep VISIBLE ===
rmgr: Heap2 len (rec/tot): 64/ 8256, tx: 0, lsn: 44B/651EDAE8, prev 44B/651EDA58, desc: VISIBLE cutoff xid 752732359 flags 1, blkref #0: rel 1663/16506/190024 fork vm blk 0 FPW, blkref #1: rel 1663/16506/190024 blk 0
rmgr: Heap2 len (rec/tot): 59/ 59, tx: 0, lsn: 44B/6B19A6E8, prev 44B/6B19A658, desc: VISIBLE cutoff xid 752734764 flags 1, blkref #0: rel 1663/16506/190024 fork vm blk 0, blkref #1: rel 1663/16506/190024 blk 0
rmgr: Heap2 len (rec/tot): 59/ 59, tx: 0, lsn: 44B/71194D40, prev 44B/71194CB0, desc: VISIBLE cutoff xid 752737874 flags 1, blkref #0: rel 1663/16506/190024 fork vm blk 0, blkref #1: rel 1663/16506/190024 blk 0
=== $ grep 190005 wals-text | grep VISIBLE ===
rmgr: Heap2 len (rec/tot): 64/ 8256, tx: 0, lsn: 44B/651F3728, prev 44B/651F36C8, desc: VISIBLE cutoff xid 752732355 flags 1, blkref #0: rel 1663/16506/190005 fork vm blk 0 FPW, blkref #1: rel 1663/16506/190005 blk 0
rmgr: Heap2 len (rec/tot): 59/ 59, tx: 0, lsn: 44B/6B19AE80, prev 44B/6B19AE20, desc: VISIBLE cutoff xid 752732667 flags 1, blkref #0: rel 1663/16506/190005 fork vm blk 0, blkref #1: rel 1663/16506/190005 blk 0
rmgr: Heap2 len (rec/tot): 59/ 59, tx: 0, lsn: 44B/6B19CF10, prev 44B/6B19AEC0, desc: VISIBLE cutoff xid 752734752 flags 1, blkref #0: rel 1663/16506/190005 fork vm blk 0, blkref #1: rel 1663/16506/190005 blk 1
rmgr: Heap2 len (rec/tot): 59/ 59, tx: 0, lsn: 44B/711954D8, prev 44B/71195478, desc: VISIBLE cutoff xid 752737869 flags 1, blkref #0: rel 1663/16506/190005 fork vm blk 0, blkref #1: rel 1663/16506/190005 blk 0

But I did not find in parsed WALs anything about vm fork for 16891.

Is it OK, or is something wrong here?

PS:
I try to remove all problematic visibility maps, with page LSN bigger than 'Minimum recovery ending
location') and start backup without it. Without it postgres started successfully.

--
Sergey Burladyan