Currently we do not allow resource conversions to change the resource quantities when updating the sorter; we only allow the metadata for their consumed resources to be modified.
However, this restricts Mesos from supporting operations that remove resources. For example, when a CSI volume with a stale profile is destroyed, it would be better to convert it into an empty resource since the disk space is no longer available. See https://issues.apache.org/jira/browse/MESOS-8825.
To make the allocator more flexible, we should allow resource conversions to remove resources when updating the sorter.