Ephemeral/Persistent disks

Basically a persistent disk == a volume

I think you are looking for the "boot from volume" feature:
http://docs.openstack.org/trunk/openstack-compute/admin/content/boot-from-volume.html

With newer versions, Cinder drivers you can create the volume from a Glance image.

An example is in DevStack:
https://github.com/openstack-dev/devstack/blob/master/exercises/boot_from_volume.sh

It is a bit more manual to create a good volume in Folsom, see this:
https://github.com/openstack-dev/devstack/blob/stable/folsom/exercises/boot_from_volume.sh

Thanks for the response,
In fact I used cinder as volume backend and followed the example above.
Once created a vm, it shows an error status without mentioning any reason in log files nova or cinder log files.
I was checking /var/lib/nova/instances to check the instance created but no new instance000000x was created in the folder.
I checked also nova show vm and as OS-EXT-SRV-ATTR:instance_name was instance-0000000a.
that means as far as I understand that the ephemeral disk is created [folder disk] in cinder-volumes level.
Could the be reason of the error ?

The instance created booted from that volume shows ERROR status in nova list instances. However I can ping it. No errors shown actually. try to figure out how it was done I could see in the /var/lib/nova/instances/instance000000x/libvirt.xml the creation of two types of disk [file and block types]
   <disk type="file" device="disk">
      <driver name="qemu" type="qcow2" cache="none"/>
      <source file="/var/lib/nova/instances/instance-00000011/disk"/>
      <target bus="virtio" dev="vda"/>
    </disk>
    <disk type="block" device="disk">
      <driver name="qemu" type="raw" cache="none"/>
      <source dev="/dev/disk/by-path/ip-172.16.0.3:3260-iscsi-iqn.2010-10.org.openstack:volume-19fd60dc-0b6e-4b11-8fc0-5cd32e45dacd-lun-1"/>
      <target bus="virtio" dev="volcinder2"/>
    </disk>

Is still something missing ?

Can you get hold of the nova-compute logs?
That should give us more help about what is going wrong.

tail -n150 /var/log/nova/nova-compute.log | grep error:

TRACE nova.compute.manager [instance: 618c913c-f47d-4449-95cb-2501fdf72bcd] libvirtError: internal error Invalid harddisk device name: volcinder2
TRACE nova.openstack.common.rpc.amqp self._set_instance_error_state(context, instance['uuid'])
TRACE nova.openstack.common.rpc.amqp libvirtError: internal error Invalid harddisk device name: volcinder2

it seems that the cinder volume not recognized by nova.
It s already configured in cinder.conf file.
To create that volume group I proceeded as follows:

dd if=/dev/zero of=volcinder2 bs=1 count=0 seek=6G
losetup /dev/loop3 volcinder2
fdisk /dev/loop3

Then we could see i cinder list the new volume group created and available.

Just after executing nova boot as follows:
nova boot --flavor 1 --image Test --key_name mykey --block-device-mapping volcinder2=19fd60dc-0b6e-4b11-8fc0-5cd32e45dacd:::0 ServerFromBoot

show building --> Error status.

???

n
p
1
ENTER
ENTER
t
8e
w

pvcreate /dev/loop3
vgcreate volcinder2 /dev/loop3

Hmm, that certainly could be a better error message.

The bit you have as "volcinder2" is meant to be a device, like "/dev/vda" (can't remember what the correct letters for "vd" are in your case.

Give that a whirl and see if it helps. You need something that libvirt (in your case) recognises as a device name.

I hope that helps get you a bit further.

I did not see how libvirt should recognise 'volcinder2' as a device. is that /dev/vda naming imporant for that ?

Thanks for your response. It helps by mentionning vda as you said before but still wondering about the 'vda' as a first disk for example will be created once the command launched ? and what is the differene between this way and using a cindervol2 as created above ?

Thanks John Garbutt, that solved my question.