Ok, I have it working. I think. The backup is running now. I used different guides from these forums and ReddIt and also the official samba and vfs_fruit man pages.
Situation:
Client: macOS Monterey 12.1
Server: Unraid 6.9.2
I have a Backups share as a staging area for servers and clients to put files before they're back-upped to the cloud. This share I made available to Apple's Time Machine with the following settings:
First, the global SMB settings in Settings > SMB:
Enhanced macOS interoperability: Yes
Enable NetBIOS: No
Second, the SMB Extras settings in Settings > SMB (stored in /boot/config/smb-extra.conf if you prefer the command line):
[Global]
min protocol = SMB2
ea support = yes
vfs objects = catia fruit streams_xattr
fruit:aapl = yes
fruit:nfs_aces = no
fruit:zero_file_id = yes
fruit:metadata = stream
fruit:encoding = native
fruit:model = MacSamba
veto files = /._*/.DS_Store/
delete veto files = yes
[Backups]
path = /mnt/user/Backups
fruit:time machine = yes
fruit:time machine max size = 3 T
In my case I used a maximum size of 3TB for the Time Machine backups. A nice extra with this configuration: a faster browsing experience of SMB shares in macOS (at least in Finder).
Note of caution: if you use the Unassigned Devices plugin, the above configuration needs to be included before '#unassigned_devices_start'.
Finally, for the Backups share, the specific SMB settings:
The Time Machine volume size limit (3TB in MB) here is probably useless as it is overridden by the SMB Extras settings.
Hope it helps some of you.