Here the results :
zfs list a lot but here are the line corresponding to backup target (each used for different test, never at the same time)
admin@truenas[~]$ sudo zfs list
NAME USED AVAIL REFER MOUNTPOINT
OnlyZpol 649G 2.88T 6.30G /mnt/OnlyZpol
OnlyZpol/CloudronBackup 256G 2.88T 256G /mnt/OnlyZpol/CloudronBackup
[...]
OnlyZpol/UserTst 133G 2.88T 66.4G /mnt/OnlyZpol/UserTst
OnlyZpol/UserTst/CloudronBck 66.4G 2.88T 66.4G /mnt/OnlyZpol/UserTst/CloudronBck
[...]
boot-pool 28.4G 68.5G 96K none
cp -aRl /path/to/snapshot /path/to/test-copy
I did the test on both backup path i have, works and is quick.
myuser@truenas:~$ cp -aRl /mnt/OnlyZpol/UserTst/CloudronBck/snapshot/ /mnt/OnlyZpol/UserTst/CloudronBck/testdir/
myuser@truenas:~$
myuser@truenas:~$ cp -aRl /mnt/OnlyZpol/CloudronBackup/snapshot/ /mnt/OnlyZpol/CloudronBackup/testdir/
myuser@truenas:~$
Current compression is LZ4. I haven't change it to ZSTD yet as previous points. Moreover i'm not sure to understand the "accepting the sshfs path" part.
So, with my limited knowledge, i'm unsure my issue come from cross-dataset references.
But i will test again without hardlink.
EDIT : as expected, same issue, remote copy fail....