Comment 4 for bug 1796825

Reviewed: https://review.openstack.org/609340
Committed: https://git.openstack.org/cgit/openstack/cinder/commit/?id=3269da58d8fb6cdbb902487914e7a6dedee8b22f
Submitter: Zuul
Branch: master

commit 3269da58d8fb6cdbb902487914e7a6dedee8b22f
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