Reload filesystem_store_datadirs configuration without service restart

In production environment, when the storage is almost full administrator will add more capacity by adding more disks, then modify the glance-api.conf and finally he/she would want to reload the configuration changes without restarting service as restarting service would break users connected to it abruptly.

For example
filesystem_store_datadirs = /x/glance/images:1
changed to
filesystem_store_datadirs = /x/glance/images:1, /y/glance/images:1

will be allowed.

But if you try to remove the existing entries, then it will give error with proper error message and continue to run with the previous configuration values.

filesystem_store_datadirs = /X/glance/images:1
changed to
filesystem_store_datadirs = /A/glance/images:1, /Y/glance/images:1

will not be allowed as users won’t be able to retrieve images stored at
/X/glance/images path.

Note:
We are depending on existing support here
https://blueprints.launchpad.net/nova/+spec/config-reload-no-restart
John Garbutt from Nova is working on above mentioned blueprint,
where he is making changes to oslo librabry for adding
"can_update_without_restart" and "update_without_restart=True"
options.
So once the above changes are in we will start implementation to address our change.