This solution can be extended to support existing block UUIDs, but it requires increased qubesdb path limit.

I think UUID is exactly on the limit:

$ echo -n /qubes-block-devices/$(uuidgen)/desc|wc -c
62

$ echo -n /qubes-block-devices/$(uuidgen)/desc|wc -c
62

It seems that you're right. I was sure that I've seen errors related to this limitation... Do you prefer full UUID or cropped version is ok?

I think internally full UUID would be better. And then at GUI/CLI made it usable (like crop to a shorter version as long as it's unambiguous).

How the device attach works here? You need to save in the qubesdb an actual device name too.

How the device attach works here? You need to save in the qubesdb an actual device name too.

I suppose, it's enough to have symlink to the actual device in the backend domain. Attach-device from libvirt should work with source /dev/${qname} or /dev/disk/by-uuid/${qname} as usual.

I think UUID is exactly on the limit:

$ echo -n /qubes-block-devices/$(uuidgen)/desc|wc -c
62

I've found what was wrong. Links in by-uuid directory are formed by blkid in ID_FS_UUID_ENC variable, which length in theory can be up to 256 characters. This is not a problem while we are stick to uuidgen and generate our own UUIDs.