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As far as I know, in order to determine if a snapshot is identical with another one or not, we should check if Received UUID of destination is equal to UUID of source snapshot or not.

But this time, this procedure can not be applied:

root@aea3:/mnt/zencefil/snapshots/rootfs# btrfs send rootfs.20170705T1334/ | pv | btrfs receive /mnt/aea3/manual-snapshots/ 
At subvol rootfs.20170705T1334/ 
At subvol rootfs.20170705T1334 
37.8GiB 0:37:27 [17.2MiB/s] [ 
root@aea3:/mnt/zencefil/snapshots/rootfs# btrfs sub show rootfs.20170705T1334/
/mnt/zencefil/snapshots/rootfs/rootfs.20170705T1334
 Name: rootfs.20170705T1334
 UUID: e66ace1b-3efc-5d40-b3ca-af000110e89d
 Parent UUID: 85bc19f1-9d61-da43-a4be-40c717aee652
 Received UUID: c6f65496-f21d-c049-b2e2-44007b269dc6
 Creation time: 2017-07-10 12:47:16 +0300
 Subvolume ID: 2561
 Generation: 4331
 Gen at creation: 4327
 Parent ID: 257
 Top level ID: 257
 Flags: readonly
 Snapshot(s):
 snapshots/rootfs/rootfs.20170710T1242
root@aea3:/mnt/zencefil/snapshots/rootfs# btrfs sub show /mnt/aea3/manual-snapshots/rootfs.20170705T1334/
/mnt/aea3/manual-snapshots/rootfs.20170705T1334
 Name: rootfs.20170705T1334
 UUID: 53bc9463-2fe6-9a41-b17d-a0a8dc5e6086
 Parent UUID: -
 Received UUID: c6f65496-f21d-c049-b2e2-44007b269dc6
 Creation time: 2017-07-12 10:36:50 +0300
 Subvolume ID: 1832
 Generation: 64698
 Gen at creation: 64627
 Parent ID: 1831
 Top level ID: 1831
 Flags: readonly
 Snapshot(s):
root@aea3:/mnt/zencefil/snapshots/rootfs# btrfs --version
btrfs-progs v4.11

However, when I create and send a test snapshot and send it on the same drives on the same machine, it works as expected:

root@aea3:/mnt/zencefil/test# btrfs send mytest1/ | pv | btrfs receive /mnt/aea3/test/
At subvol mytest1/
At subvol mytest1
 540 B 0:00:00 [73.1KiB/s] [ ]
root@aea3:/mnt/zencefil/test# btrfs sub show mytest1/
/mnt/zencefil/test/mytest1
 Name: mytest1
 UUID: 3a932406-8a81-5047-9a23-9333564752c2
 Parent UUID: -
 Received UUID: -
 Creation time: 2017-07-12 11:41:22 +0300
 Subvolume ID: 2567
 Generation: 4369
 Gen at creation: 4367
 Parent ID: 5
 Top level ID: 5
 Flags: readonly
 Snapshot(s):
root@aea3:/mnt/zencefil/test# btrfs sub show /mnt/aea3/test/mytest1/
/mnt/aea3/test/mytest1
 Name: mytest1
 UUID: 98d59473-5a47-7742-bcf7-47262b9cada2
 Parent UUID: -
 Received UUID: 3a932406-8a81-5047-9a23-9333564752c2
 Creation time: 2017-07-12 11:44:16 +0300
 Subvolume ID: 1844
 Generation: 64738
 Gen at creation: 64737
 Parent ID: 5
 Top level ID: 5
 Flags: readonly
 Snapshot(s):

What is the missing point here?

In the first case the snapshot /mnt/zencefil/snapshots/rootfs/rootfs.20170705T1334 has been received from an other location before. The original UUID is stored as Received UUID. When transmitting this snapshot again this Received UUID is also used for the copy.

Therefore in order to determine if a snapshot is identical with (that is copy of) another, check if Received UUID of destination is equal to Received UUID of source, if present, or UUID of source, otherwise.