Comment 2 for bug 1890833

Reviewed: https://review.opendev.org/745402
Committed: https://git.openstack.org/cgit/openstack/manila/commit/?id=22d6fe98a3f437709901fac4e4ec65fec414f7d0
Submitter: Zuul
Branch: master

commit 22d6fe98a3f437709901fac4e4ec65fec414f7d0
Author: Goutham Pacha Ravi <email address hidden>
Date: Fri Aug 7 12:38:05 2020 -0700

    Fix capacity calculations in the CephFS driver

    The driver inflated total and available capacity
    due to an incorrect calculation. The driver was
    also ignoring the configuration option
    "reserved_share_percentage" that allows
    deployers to set aside space from scheduling
    to prevent oversubscription.

    While this bugfix may have an upgrade impact,
    some things must be clarified:
    - Inflating the total, free space will allow
      manila to schedule workloads that may run
      out of space - this may cause end user
      downtime and frustration, because shares are
      created (empty subvolumes on ceph occupy no
      space) easily, but they could get throttled
      as they start to fill up.
    - CephFS shares are always thinly provisioned
      but, the driver does not support oversubscription
      via manila. So, real free space is what
      determines capacity based scheduler decisions.
      Users however expect share sizes to be honored,
      and manila will allow provisioning as long
      as there is free space on the cluster. This
      means that Ceph cluster administrators
      must manage oversubscription outside of manila
      to prevent misbehavior.

    Depends-On: Ic96b65d2caab788afca8bfc45575f3c05dc88008
    Change-Id: I6ab157d6d099fe910ec1d90193783b55053ce8f6
    Closes-Bug: #1890833
    Signed-off-by: Goutham Pacha Ravi <email address hidden>