I understand. Next time I write everything in the mail body.

I deleted the snapshot on the destination side. So, I had to do again
just now resulting in same problem:

   btrfs send -p | \
       "${SRC}/@2021-10-30-095620_files" \
       "${DST}/@2021-10-30-100526_files" | \
       btrfs receive "${DST}/"; echo $status; sync
   At subvol /mnt/backup/src/@00_snapshot_hourly/@2021-10-30-100526_files
   At snapshot @2021-10-30-100526_files
   0

The parent snapshot:

   bash-5.1# btrfs subvolume show "${SRC}/@2021-10-30-095620_files"
   WARNING: the subvolume is read-write and has received_uuid set,
   	 don't use it for incremental send. Please see section
   	 'SUBVOLUME FLAGS' in manual page btrfs-subvolume for
   	 further information.
   @00_snapshot_hourly/@2021-10-30-095620_files
   	Name: 			@2021-10-30-095620_files
   	UUID: 			c4914d2e-b1f7-134e-984f-bf686f8d919b
   	Parent UUID: 		537c7db2-05da-6546-9092-89f873d7276a
   	Received UUID: 		7eb8ade9-0234-474c-b3c6-
   3b439bb51aa4
   	Creation time: 		2021-10-30 11:56:20 +0200
   	Subvolume ID: 		5914
   	Generation: 		319458
   	Gen at creation: 	319458
   	Parent ID: 		5855
   	Top level ID: 		5855
   	Flags: 			readonly
   	Send transid: 		14924
   	Send time: 		2021-10-30 11:56:20 +0200
   	Receive transid: 	1276
   	Receive time: 		2020-10-30 23:44:11 +0100
   	Snapshot(s):

Snapshot that needs to be sent:

   bash-5.1# btrfs subvolume show "${SRC}/@2021-10-30-100526_files"
   WARNING: the subvolume is read-write and has received_uuid set,
   	 don't use it for incremental send. Please see section
   	 'SUBVOLUME FLAGS' in manual page btrfs-subvolume for
   	 further information.
   @00_snapshot_hourly/@2021-10-30-100526_files
   	Name: 			@2021-10-30-100526_files
   	UUID: 			74c54234-20b3-7d4c-9dd8-ebb919d2e876
   	Parent UUID: 		537c7db2-05da-6546-9092-89f873d7276a
   	Received UUID: 		7eb8ade9-0234-474c-b3c6-
   3b439bb51aa4
   	Creation time: 		2021-10-30 12:05:26 +0200
   	Subvolume ID: 		5920
   	Generation: 		319481
   	Gen at creation: 	319481
   	Parent ID: 		5855
   	Top level ID: 		5855
   	Flags: 			readonly
   	Send transid: 		14924
   	Send time: 		2021-10-30 12:05:26 +0200
   	Receive transid: 	1276
   	Receive time: 		2020-10-30 23:44:11 +0100
   	Snapshot(s):


Resulting snapshot on the destination side:

   bash-5.1# btrfs subvolume show "${DST}/@2021-10-30-100526_files"
   WARNING: the subvolume is read-write and has received_uuid set,
   	 don't use it for incremental send. Please see section
   	 'SUBVOLUME FLAGS' in manual page btrfs-subvolume for
   	 further information.
   @00_snapshot_hourly/@2021-10-30-100526_files
   	Name: 			@2021-10-30-100526_files
   	UUID: 			4a97faed-5852-e140-a6ec-4eaac6ab4863
   	Parent UUID: 		e04d86db-293a-cc44-a226-059677aa6e11
   	Received UUID: 		7eb8ade9-0234-474c-b3c6-
   3b439bb51aa4
   	Creation time: 		2021-10-30 15:54:32 +0200
   	Subvolume ID: 		3300
   	Generation: 		1146
   	Gen at creation: 	1146
   	Parent ID: 		1988
   	Top level ID: 		1988
   	Flags: 			readonly
   	Send transid: 		319434
   	Send time: 		2021-10-30 15:54:32 +0200
   	Receive transid: 	1147
   	Receive time: 		2021-10-30 15:54:32 +0200
   	Snapshot(s):

-- 
My GnuPG public key:
gpg --auto-key-locate clear,dane --locate-external-key d@duxsco.de



On Sat, 2021-10-30 at 16:38 +0300, Andrei Borzenkov wrote:

This makes it impossible to comment on your description or logs inline.

Anyway, the usual primary suspect is duplicated received UUID. Show
"btrfs subvolume show snapshot" on both source and destination systems
for each involved snapshot.