LXD 3.21: cannot attach ISO to VM in cluster

Using LXD 3.21 in a cluster with ceph for backing disk, except for the ISO.

Trying to attach an ISO to a VM like so:

ubuntu@aa1-cptef101-n4:~$ lxc config device add debian9 iso disk source=/opt/boot-        
imgs/debian-9.12.0-amd64-netinst.iso
Device iso added to debian9
ubuntu@aa1-cptef101-n4:~$ file /opt/boot-imgs/debian-9.12.0-amd64-netinst.iso
/opt/boot-imgs/debian-9.12.0-amd64-netinst.iso: DOS/MBR boot sector; partition 2 : 
ID=0xef, start-CHS (0x3ff,254,63), end-CHS (0x3ff,254,63), startsector 3784, 832 sectors
ubuntu@aa1-cptef101-n4:~$ ls -l /opt/boot-imgs/debian-9.12.0-amd64-netinst.iso
-rwxrwxrwx 1 root root 306184192 Feb  9 03:38 /opt/boot-imgs/debian-9.12.0-amd64- 
netinst.iso    netinst.iso
+---------+---------+------+------+-----------------+-----------+-----------------+
|  NAME   |  STATE  | IPV4 | IPV6 |      TYPE       | SNAPSHOTS |    LOCATION     |
+---------+---------+------+------+-----------------+-----------+-----------------+
| debian9 | STOPPED |      |      | VIRTUAL-MACHINE | 0         | aa1-cptef101-n4 |
+---------+---------+------+------+-----------------+-----------+-----------------+

The error output is:
root@aa1-cptef101-n4:/home/ubuntu# lxc start debian9
Error: Failed to start device “iso”: Cannot find disk source

Does the ISO need to be on ceph as well?

https://github.com/lxc/lxd/pull/6906 should fix that

Yep - that fixed that issue, thanks!