Comment 8 for bug 1796825

Reviewed: https://review.openstack.org/639754
Committed: https://git.openstack.org/cgit/openstack/cinder/commit/?id=c5e2ed9daf2be132819755f7edaea5c29b1dca4c
Submitter: Zuul
Branch: stable/rocky

commit c5e2ed9daf2be132819755f7edaea5c29b1dca4c
Author: Ryan Liang <email address hidden>
Date: Wed Oct 10 17:24:34 2018 +0800

    VNX: Add constraints for async migration

    Async migration was introduced to accelerate the cloning of volume (
    The cloned volume can be accessed after the migration starts, no need
    to wait it for finishing).

    However, before the migration finishes, snapshots cannot be created from
    the source volume any more, which could affect subsequent clones from
    the same source volume. That is the constraint of async migration.

    The fix is adding the constraint to the doc of VNX driver. And give
    users some advises on how to make the subsequent volume clone work.

    Change-Id: I6ad3c6b967d5a33966b40c3a54c0f110576c8d87
    Closes-bug: #1796825
    (cherry picked from commit 3269da58d8fb6cdbb902487914e7a6dedee8b22f)