Open Stack

On Thu, 2022-06-16 at 11:08 +0430, fereshteh loghmani wrote:
> hello
> I have a question about disk format in OpenStack.
> I created a new server and shut off that server and with the command
> qemu-img info disk I found my "disk format" in qcow2 and it shows the
> "backing file format: raw".
> could you tell me why these 2 are different?
your nova config options and also perfromance
i belve have in a raw backing file is faster but it uses more space
https://docs.openstack.org/nova/latest/configuration/config.html#DEFAULT.force_raw_images
that defaults to true so we will use raw images for backing files

https://docs.openstack.org/nova/latest/configuration/config.html#libvirt.images_type
default to default meaning the default is virt driver defiened for libvirt the default is qcow

https://docs.openstack.org/nova/latest/configuration/config.html#DEFAULT.use_cow_images
defaults to true
which allows qcow backing files to be used but the force flag overrides that.

so if you disabel force_raw_images and your image in glance is qcow2 then the backing file would be qcow2
if the image was raw in glance the backing file woudl be raw.


from a performance vs disk usage point of view i belive our defaults of using a raw backing file which is
shared between multiple vms if they use the same image and qcow for the vm itself is correct

if you want higher performce then the defautl then you can you can set images_type to raw

if you want disbale the use of backing files you can set the images_type to flat

so the answer basicaly is because that is what the config file currently has selected
and you can change that behavor if you want too.

> (I use OpenStack without ceph and without volumes.)
> thank you