We have upgraded from commit f2188 to cb6cf and have 3 hosts. With none of these hosts backups worked anymore after doing the upgrade. When reverting back to f2188 all works again.
We also deleted orphaned VDIs and let the garbage collector do its job but it did not helped.

Hosts/Errors :
1.) XCP-ng 8.2.1 : "VDI must be free or attached to exactly one VM"
2.) XCP-ng 8.2.1 (with latest updates): "VDI must be free or attached to exactly one VM"
3.) XenServer 7.1.0 : "MESSAGE_METHOD_UNKOWN(VDI.get_cbt_enabled)

With the Xenserver 7.1.0 CBT is not enabled (and can not be enabled).

It is not fully clear to me if "full backup interval" is really needed (for preventing corrupted backups over the time) with delta backups wich are supposed to be "continous" and which are merging in new deltas automatically.

So is it really suggested to set a full backup interval with continous delta backups ?

And what means that in practice for disk usage ? If we have for example an retention of 3 and a full backup intervall of 5. Will it mean :
1st backup : Full
2nd backup : Delta
3rd backup : Delta
4th backup : Delta (now the 2nd delta gets new "full")
5th backup : Full (now 3rd delta becomes another "full" and we have 2 fulls ?)

I am confused. Maybe you could help me understand this mechanism better.