Comment 5 for bug 1697543

Reviewed: https://review.openstack.org/479012
Committed: https://git.openstack.org/cgit/openstack/swift/commit/?id=aa82d2cba82209f1bf3944c6d2a67965af5a1540
Submitter: Zuul
Branch: master

commit aa82d2cba82209f1bf3944c6d2a67965af5a1540
Author: Samuel Merritt <email address hidden>
Date: Thu Jun 29 10:23:38 2017 -0700

    Save ring builder if dispersion changes

    There are cases where a rebalance improves dispersion, but doesn't
    improve balance. This is because the balance of a ring builder is
    taken to be the balance of its least-balanced device, so if there's a
    device that has no partitions, wants some, but can't get them, then
    we'll never save the ring builder even if every other device in the
    ring got better.

    We can detect this situation by looking at the dispersion number; if it
    changes, then the rebalance needs to be saved in order to continue to
    make progress.

    Partial-Bug: #1697543

    Change-Id: Ie239b958fc7e0547ffda2bebf61546bd4ef3d829