Updating this with a few solutions to problems I had and a small request to update the documentation about snapshots.
As you can se above in my previous post I have two partitions in btrfs and one in ext4. The reason I want the /media/diverse in ext4 is because I read somewhere that downloading torrent files to a btrfs partition could be hurtful for btrfs so I opted for ext4 (not backing up that directory).
Finding information about how to get this work was NOT easy. For setting the backupfolder mode to btrfs is described in the documentation, easy peasy.
But when it comes to snapshotting all info (if any at all) had to be found on this forum.
Quite a few times I found forum posts with questions about the exact things I needed answers to but those posts were all unanswered. 
Since I installed the client with “no snapshot” I had to find out how to change that.
I figured out I need to remove/rename /usr/local/etc/urbackup/no_filesystem_snapshot and create a file called snapshot.cfg.
I read about this file in forum posts and figured out that all the snapshot scripts are in /usr/local/share/urbackup/
$ ls /usr/local/share/urbackup/ -l
total 48
-rwxr-xr-x 1 root root 732 Oct 18 14:41 btrfs_create_filesystem_snapshot
-rwxr-xr-x 1 root root 662 Oct 18 14:41 btrfs_remove_filesystem_snapshot
-rwxr-xr-x 1 root root 3447 Oct 18 14:41 dattobd_create_snapshot
-rwxr-xr-x 1 root root 2332 Oct 18 14:41 dattobd_remove_snapshot
-rwxr-xr-x 1 root root 5145 Oct 18 14:41 dm_create_snapshot
-rwxr-xr-x 1 root root 2191 Oct 18 14:41 dm_remove_snapshot
-rwxr-xr-x 1 root root 378 Oct 18 14:41 filesystem_snapshot_common
-rwxr-xr-x 1 root root 1784 Oct 18 14:41 lvm_create_filesystem_snapshot
-rwxr-xr-x 1 root root 1869 Oct 18 14:41 lvm_remove_filesystem_snapshot
drwx------ 2 root root 4096 Oct 18 14:41 scripts
-rw-r--r-- 1 root root 436 Oct 18 14:41 urbackup_ecdsa409k1.pub
I created the file /usr/local/etc/urbackup/snapshot.cfg and tried all of the different modes but they all gave different error messages because of the different filesystems.
I used nano to try to understand the scripts and try to figure out how to set different modes to different disks but could not figure it out. Searching the internet about this subject rendered absolutely nothing.
#This is a key=value config file for determining the scripts/programs to create snapshots
create_filesystem_snapshot=/usr/local/share/urbackup/btrfs_create_filesystem_snapshot
remove_filesystem_snapshot=/usr/local/share/urbackup/btrfs_remove_filesystem_snapshot
I spent quite a few days on trying around with different scripts in snapshot.cfg but could not get it to work.
I did not create an lvm on the ext4 (because I didn’t know about these mechanics at all before starting this project) and wasn’t too keen on shrinking the filesystem, spending space just for snapshots on that drive since it only contains media files, they most probably wont change when the backup is run and if any does, it’s not really a big deal if that file fails on one backup (backup wont fail), a new incremental backup is run in 6hrs.
So what made sense to me was to use the btrfs_create filesystem_snapshot and btrfs_remove_filesystem_snapshot on my /media/users directory because those files WILL change during backup.
But this produced this log entry for every backup
Level Time Message
Errors 10/28/22 12:44 Creating snapshot of "/media/diverse" failed
Errors 10/28/22 12:44 Cannot create snapshot of file system with type ext4
Errors 10/28/22 12:44 Creating snapshot of "diverse" failed.
So I simply want to exclude the drive from snapshotting.
When for the hundread time I read the contents of /usr/local/share/urbackup/ I suddenly realized there is a reference to /etc/urbackup/exclude_snapshot_mountpoints in the file filesystem_snapshot_common. It’s the same directory used to set the backupfolder to btrfs but the file was not there.
After searching the forum for exclude_snapshot_mountpoints I finally found my answer. Simply create /etc/urbackup/exclude_snapshot_mountpoints and put /media/diverse, ie the mountpoint you want to exclude, in that file.
$ sudo echo "/media/diverse" > /etc/urbackup/exclude_snapshot_mountpoints
This should really be mentioned in the documentation, I can’t be the only one with that problem and it really was an easy fix, if you just know how to do it.
Question:
When searching for info on /usr/local/etc/urbackup/snapshot.cfg I read in ONE post that it included “volumes_mounted_locally=0”. I have the client and the server on the same machine, should I add this to the file?
#This is a key=value config file for determining the scripts/programs to create snapshots
create_filesystem_snapshot=/usr/local/share/urbackup/btrfs_create_filesystem_snapshot
remove_filesystem_snapshot=/usr/local/share/urbackup/btrfs_remove_filesystem_snapshot
volumes_mounted_locally=1
Or maybe even volumes_mounted_locally=2 or even 3?
Both drives I backup are locally mounted but only one use snapshots. And the sdcard the rpi is running on is also obv mounted locally but I don’t back anything up from that drive.
As of now, volumes_mounted_locally is not in my file. Should it?
Maybe this post turns up in other peoples search having the same problems as me.
Still hoping for some input/answers on my first post. 