It’s possible to downscale resources of a virtual server with the existing virtualization technologies. Removing or adding RAM, CPU or pluggable other HW from virtual server can be done without problem.
However, while downscaling disc can be done at virtual level, it is likely to have a problem at operating system level.
We would like to explain with an example:
Suppose we have a virtual server with 100GB disc and out of this 60GB is being used. Systems use specific algorithms while writing on the disc however writing is done randomly on disc, hence the 60GB data can be spreaded anywhere over the whole disc. If we downscale the disc to 60GB from 100GB, there is high risk of loosing some of the data since we don’t know the exact location of data on the disc.
This is one of the reasons of loosing data only. If the disc structure of the virtual server is changed at the operating system level after the initial setup, this downscaling may even cause not re-starting of the virtual server.
There are some programs available to minimise such risks however none of them guarantees %100 success.
Due to above explained reasons OpenStack developers and community members and also as SkyAtlas we do not advise resource downscaling.