Hello stackers,


It has come to our attention recently that nova, cinder, and glance
treat image properties differently. This came up in a discussion on a
patch to extend the allowable length of image metadata values in cinder [0].


Briefly:

   - glance allows image property values to be unrestricted in the
schema, but effectively limited to 65535 bytes by the database.

   - cinder restricts the length of an image property value to 255 chars
(by request schema) if you try to update the volume_image_properties on
a volume, but if you create a volume from an image, cinder happily
copies over all the image properties from glance, giving them an
effective limit of 65535 bytes.

   - nova mostly truncates image properties values to 255 chars (it's a
bit more complicated, but we can consider that to be nova's behavior for
the purposes of this discussion).

(We're not sure what ironic does, but they're in the subject line as a
consumer of images and volumes.)

Note that we're talking about metadata *values* here; the metadata keys
are already restricted by charset to basically US-ASCII and 255 length.
Values are allowed to be unicode.

The point of this email is that it would be good for the services to be
consistent about image metadata values. Thus, we propose that all
services should restrict image metadata values to 255 unicode
characters. (Which is sort of happening already, but not consistently.)

The further point of this email is to find out what the impact of this
restriction would be on current users. So please respond if you have a
use case that would be affected by this change, and could not be
addressed by, for example, instead of having one enormous value for a
single key, the metadata was broken into a number of key/value pairs
that would satisfy the 255 char restriction.


Thanks for your attention to this!


[0] https://review.opendev.org/c/openstack/cinder/+/868485