Comment 7 for bug 1922408

Reviewed: https://review.opendev.org/c/openstack/cinder/+/803181
Committed: https://opendev.org/openstack/cinder/commit/165122557d4976ae902ac6156d151214fec037ae
Submitter: "Zuul (22348)"
Branch: stable/wallaby

commit 165122557d4976ae902ac6156d151214fec037ae
Author: haixin <email address hidden>
Date: Sat Apr 3 12:36:14 2021 +0800

    [rbd] Fix create encrypted volume from snapshot

    Usually the source volume would be the same size or smaller
    than the destination volume and they must share the same
    volume-type. In particular, when the destination volume is
    same size as the source volume, creating an encrypted volume
    from a snapshot of an encrypted volume truncates the data in
    the new volume.

    In order to fix this the RBD workflow would be something
    like this:
    A source luks volume would be 1026M, we write some data
    and create a snap from it. We like to create a new luks
    volume from a snapshot so the create_volume_from_snapshot()
    method performs a RBD clone first and then a resize if needed.

    In addition the _clone() method creates a clone
    (copy-on-write child) of the parent snapshot. Object size
    will be identical to that of the parent image unless specified
    (we don't in cinder) so size will be the same as the parent
    snapshot.

    If the desired size of the destination luks volume is 1G the
    create_volume_from_snapshot() won't perform any resize and
    will be 1026M as the parent. This solves bug #1922408 because
    we don't force it to resize and because of that we don't
    truncate the data anymore.

    The second case scenario is when we would like to increase
    the size of the destination volume. As far as I can tell this
    won't face the encryption header problem but we still need to
    calculate the difference size to provide the size that the
    user is expecting.

    That's why the fix proposed calculate the new_size based on:
    size difference = desired size - size of source volume
    new size = current size + size difference

    Closes-Bug: #1922408
    Co-Authored-By: Sofia Enriquez <email address hidden>
    Change-Id: I220b5e3b01d115262a8b1dd45758f0531aea0edf
    (cherry picked from commit cf1c5252965e731d77af503529ca42e269c305c6)