Your glance images need to be raw, also. A QCOW image will be copied/converted.

On 2/8/2016 3:33 PM, Jason Dillaman wrote:
If Nova and Glance are properly configured, it should only require a quick 
clone of the Glance image to create your Nova ephemeral image.  Have you 
double-checked your configuration against the documentation [1]?  What version 
of OpenStack are you using?

To answer your questions:

- From Ceph point of view. does COW works cross pool i.e. image from glance
pool ---> (cow) --> instance disk on nova pool
Yes, cloning copy-on-write images works across pools

- Will a single pool for glance and nova instead of separate pool . will help
here ?
Should be no change -- the creation of the clone is extremely lightweight (add 
the image to a directory, create a couple metadata objects)

- Is there any tunable parameter from Ceph or OpenStack side that should be
set ?
I'd double-check your OpenStack configuration.  Perhaps Glance isn't configured with 
"show_image_direct_url = True", or Glance is configured to cache your RBD 
images, or you have an older OpenStack release that requires patches to fully support 
Nova+RBD.

[1] http://docs.ceph.com/docs/master/rbd/rbd-openstack/

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