This can be possible. But before continuing we need to understand about the disk concept. In the virtual servers there is root disk and external disk. Root disk is nothing but where operating system resides. From openstack point of view both disks are volumes. The root disk cannot be increased on the fly. But the we can take snapshot of the root disk and create a volume from the snapshot. Later we can launch instance from this volume with a desired disk size. Regarding external volume, its straight forward just increase the volume size by web gui or through the API. But keep in mind that, if the data is continuously writing into this volume, there is a chance for data corruption. So, we advice to make the volume, detach from the instance, take it offline and increase the size.