Comment 8 for bug 1988942

as i noted on the patch the limit comes form nova not glance.

nova dropped support for custom image properties in 2015

when we converted images to be modelled as oslo versioned objects in
https://github.com/openstack/nova/commit/40c2a4d6b05db879f976fc7eaf7a1a5aa93b270c

as part of the request spec object spec
https://specs.openstack.org/openstack/nova-specs/specs/mitaka/implemented/request-spec-object-mitaka.html

nova stores image properties in the isntance_system_metadata db table with a max value length of 255

https://github.com/openstack/nova/blob/9dceb735f6926c71a8c2e2c6ef8b5e0a1363041d/nova/db/main/models.py#L945-L946

volume metadata may exceed 255 but each image metadata value may not exceed 255