Currently in our Cloud we are using a gluster storage for cinder and glance.
For nova we are using a shared file system (implemented using gluster) for
part of the compute nodes; the rest of the compute nodes use the local disk.
We are now planning the replacement of gluster with ceph. The idea is
therefore to use ceph for cinder, glance. Ceph would be used for nova but
just for a set of compute nodes (the other compute nodes would keep using
the local disk).
In such configuration I see a problem with the choice of the best format
As far as I understand (please correct me if am wrong) the ideal setup
would be using raw images for VMs targeted to compute nodes using ceph, and
qcow2 images for VMs targeted to compute nodes using the local disk for
In fact starting a VM using a qcow2 image on a compute node using ceph for
nova works but it is quite inefficient since the qcow2 image must be first
downloaded in /var/lib/nova/instances/_base and then converted into raw.
This also means that some space is needed on the local disk.
And if you start a VM using a raw image on a a compute node using the local
disk for nova, the raw image (usually quite big) must be downloaded on the
compute node, and this is less efficient wrt a qcow2 image. It is true that
the qcow2 is then converted into raw, but I think that most of the time is
taken in downloading the image.
Did I get it right ?
Any advice ?
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