Samba recently added the ability to host time-machine backups over a local network with version 4.8. I set up an old Seagate goflex with Arch linux and followed this guide to get samba running. It's working except that every time a backup is started on either of my two laptops I get a warning that "The identity of the backup disk “ArchBackups” has changed since the previous backup." and have to click through. Any idea why my mac isn't trusting the backup server? There is this old question but that seems to be a more intermittent problem on an older version of OSX and never got a satisfactory answer.
Edit: Find my solution below.
I have the same problem on Raspian with a Ext4 file system (no LVM). Samba is version 4.8.5, compiled from source by myself. MacOS is High Sierra, backup is encrypted.
Time Machine pretends to have finished the backup successfully. When it starts the next time, the message about the changed identity appears. After choosing "backup anyway", it creates a completely new backup in a new sparse file. (Like @thaimin described it).
On the Samba mailing list someone states:
Recompile samba with
--enable-spotlightand all works.
That is not true in my case, because I did compile Samba 4.8.5 with this option (what I didn't do is setting up tracker as suggested in this blog post).
I compiled Samba in the previous version (4.8.4). Now it works as expected (same system, same dependencies, same configuration).
Someone already created a bug ticket for Samba.