To change RAM size, VDISK size or VCPU count of an openstack instance you have to use nova resize. You can't change for e.g. just the RAM size with a parameter, you have to assign a new/other flavor. If there is no suitable flavor for the new properties of the VM, create a new one.

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nova resize [--poll] <server> <flavor>

The resize could take a while, after it is finished, the VM boots up with the new specifications. SSH into the VM and check if everything is alright. If so -> use nova resize-confirm <server> to get rid of the old VM image and keep the new one. If you run into any trouble you can use nova-resize-revert <server> to drop the new VM image and use the old one.

Please mind the following:

• openstack will shutdown the VM before the resize, so you have to acknowledge a downtime
• be careful if you resize a vdisk to a smaller value, as there is the risk of losing data
• as openstack recreates the image with qemu-img convert, a resize can take a long time (depending on the size of the disk)