Enable cloning for rbd-backed ephemeral disks

[OpenStack Infra]

https://review.openstack.org/94295
commit 112b140e2daa7207a8d37c77a92456b155f3ecb9
Author: Dmitry Borodaenko <email address hidden>
Date: Mon May 19 16:30:14 2014 -0700

    Enable cloning for rbd-backed ephemeral disks

    Currently when using rbd as an image backend, nova downloads the glance
    image to local disk and then copies it again into rbd. This can be very
    slow for large images, and wastes bandwidth as well as disk space.

    When the glance image is stored in the same ceph cluster, the data is
    being pulled out and pushed back in unnecessarily. Instead, create a
    copy-on-write clone of the image. This is fast, and does not depend on
    the size of the image. Instead of taking minutes, booting takes seconds,
    and is not limited by the disk copy.

    Add some rbd utility functions from cinder to support cloning and let
    the rbd imagebackend rely on librbd instead of the rbd command line tool
    for checking image existence.

    Adds a new clone() method to the image backend, so backends like rbd can
    make optimizations like this. Try to use clone() for the root disk when
    it comes from an image, but fall back to fetch_to_raw() if clone()
    fails.

    Instead of calling disk.get_disk_size() directly from
    verify_base_size(), which assumes the disk is stored locally, add a new
    method that is overridden by the Rbd subclass to get the disk size.

    DocImpact

    Implements: blueprint rbd-clone-image-handler
    Closes-Bug: 1226351
    Co-Authored-By: Josh Durgin <email address hidden>
    Signed-Off-By: Josh Durgin <email address hidden>
    Signed-Off-By: Zhi Yan Liu <email address hidden>
    Signed-Off-By: Dmitry Borodaenko <email address hidden>
    Change-Id: I0f50659b54a92fc21086990be8925ea15008569a

[Atsushi SAKAI (sakaia)]

From seeing this.

This is not affects to API-SITE.

https://github.com/openstack/nova-specs/blob/master/specs/juno/implemented/rbd-clone-image-handler.rst