Cannot send btrfs incremental snapshots to external disk

[oldgeek]

I am unable to send incremental snapshots to my external HD. The situation is this. Recently I re-installed Kubuntu 24.04 as a clean install, then sent the @home subvolume to my internal HD, without problems (I didn't forget to change the read only to read/write). I didn't change the @ subvolume. The system has been working fine, even better than before. I have taken two snapshots of each subvolume (@ and @home). Tonight I tried to send them as an incremental snapshot for each, but got the following messages:

ERROR: snapshot: cannot find parent subvolume e49f38e4-7ecd-cd49-8bf7-9a88c2e4ea3e (for the @ incremental snap)
ERROR: snapshot: cannot find parent subvolume 1fcd27d2-0cca-f247-a0e4-6fe36f1a027c (for the @home one)

I must add that none of the snapshots I took after re-installing were sent to the external hard disk. Is that the problem? Should I have sent my first snapshots of @ and @home to the external HD before trying an incremental send?

[oldgeek]

I think I have solved my own problem. I sent my first @ snapshot to the external HD, and then tried the incremental snapshot command again, and it's working. I guess the parent subvolume is the most recent one sent to the HD, and the set of numbers and letters following is its identifier (UUID?). Anyway, it makes sense to me, and now I understand this aspect of btrfs a little better. I'll mark this SOLVED once I'm sure it worked.

[jlittle]

Should I have sent my first snapshots of @ and @home to the external HD before trying an incremental send?

Yes. For incremental send/receive to work, both source and destination have to have matching parent subvolumes. It's easy to break the match.