Issue with sending incremental backups

[mcamou]

I am setting up btrbk. The first backup went fine. However, subsequent backups error out when sending the snapshot:

Checking for missing backups of "/var/lib/_btrbk_snap/home.*" in: phlogiston:/volume1/TARDIS/tumbolia/
Creating incremental backup...
[send/receive] target: phlogiston:/volume1/TARDIS/tumbolia/home.20250507T1738
[send/receive] source: /var/lib/_btrbk_snap/home.20250507T1738
[send/receive] parent: /var/lib/_btrbk_snap/home.20250507T1201
[send/receive] checking target metadata: phlogiston:/volume1/TARDIS/tumbolia/home.20250507T1738
ERROR: Failed to send/receive subvolume: /var/lib/_btrbk_snap/home.20250507T1738 [/var/lib/_btrbk_snap/home.20250507T1201] -> phlogiston:/volume1/TARDIS/tumbolia/home.20250507T1738
ERROR: ... failed to check target subvolume: phlogiston:/volume1/TARDIS/tumbolia/home.20250507T1738
ERROR: ... Command execution failed (exitcode=1)
ERROR: ... sh: ssh -i '/home/mario/.ssh/id_rsa' mario@phlogiston.tegu 'sudo -n btrfs subvolume show '\''/volume1/TARDIS/tumbolia/home.20250507T1738'\'''
ERROR: ... cannot find real path for '/volume1/TARDIS/tumbolia/home.20250507T1738': No such file or directory
ERROR: Error while resuming backups, aborting

...T1201 is the full snapshot, ...T1738 is the incremental one. It would seem that it's checking whether the incremental snapshot exists on the remote, but instead of sending it over if it doesn't it just errors out. I'm probably doing something wrong.

Here's my btrbk.conf (comments removed):

transaction_log            /var/log/btrbk.log
ssh_identity               /home/mario/.ssh/id_rsa
ssh_user                   mario

backend_local_user         btrfs-progs-sudo
backend_remote             btrfs-progs-sudo

stream_buffer              256m

snapshot_dir               /var/lib/_btrbk_snap

snapshot_create            always

incremental                yes

preserve_hour_of_day       1

preserve_day_of_week       monday

stream_compress            zstd
stream_compress_level      10
stream_compress_threads    4

lockfile                   /var/lock/btrbk.lock

snapshot_preserve_min   2d
snapshot_preserve       14d 10w *m

target_preserve_min     15d
target_preserve         20d 10w *m

target       ssh://phlogiston/volume1/TARDIS/tumbolia
subvolume    /sysroot
subvolume    /var
subvolume    /home

One possible issue is that the remote is a Synology NAS which is running btrfs, but uses btrfs-progs v4.0.