[Bug 1940043] Re: Upgrade from OVN 20.03 to newer OVN version will cause data plane outage

[OpenStack Infra]

Reviewed:  https://review.opendev.org/c/x/charm-ovn-dedicated-chassis/+/804744
Committed: https://opendev.org/x/charm-ovn-dedicated-chassis/commit/33ed06cb238ea6b7f031a6500c054d522d538d88
Submitter: "Zuul (22348)"
Branch:    master

commit 33ed06cb238ea6b7f031a6500c054d522d538d88
Author: Frode Nordahl <frode.nordahl at canonical.com>
Date:   Mon Aug 16 15:27:46 2021 +0200

    Improve handling of major version upgrades
    
    Setting the external_ids:ovn-match-northd-version value to
    'true' will make the ovn-controller refrain from making updates to
    the data plane tables in the event of a version mismatch.
    
    This in combination with stopping the ovn-controller with the
    ovn-ctl stop_controller --restart command will allow upgrades
    to progress with little or no data plane downtime.
    (Note that we will accomplish this by a separate proposal to the
    OVN package itself in Ubuntu.)
    
    As soon as the central components are upgraded ovn-controller will
    notice and resume (re-)programming of the local Open vSwitch data
    plane.
    
    Closes-Bug: #1940043
    Change-Id: I16fedbc455e25bec0de4a475a9daa55b700ab3a0


** Changed in: charm-ovn-dedicated-chassis
       Status: In Progress => Fix Committed

** Changed in: charm-ovn-chassis
       Status: In Progress => Fix Committed

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https://bugs.launchpad.net/bugs/1940043

Title:
  Upgrade from OVN 20.03 to newer OVN version will cause data plane
  outage

Status in charm-layer-ovn:
  Fix Committed
Status in charm-ovn-chassis:
  Fix Committed
Status in charm-ovn-dedicated-chassis:
  Fix Committed
Status in ovn package in Ubuntu:
  In Progress
Status in ovn source package in Focal:
  New
Status in ovn source package in Hirsute:
  New
Status in ovn source package in Impish:
  In Progress

Bug description:
  The upstream recommendation for upgrades of OVN is to first upgrade
  the data plane components (chassis aka. ovn-controller), and then
  upgrade the central components (the database schema and ovn-northd).
  The rationale for this is that the new version of the ovn-controller
  is required to cope with any changes to database schema or how northd
  programs flows.

  However, during the course of rapid OVN development there has also
  been introduced changes that make the new ovn-controller not cope with
  a old database schema, breaking the recommended upgrade procedure.

  To cope with this upstream has introduced a new optional configuration
  for the ovn-controller that allows it to detect version
  inconsistencies, and when they are present stop it from making changes
  to the data plane until the version inconsistency is resolved [0].

  For the above mentioned configuration to be effective we also need the
  package to call ``ovn-ctl stop_controller`` with the --restart option
  so that the ovn-controller does not flush the installed flows on exit.

  We should make required changes to packages and charms to allow
  upgrades to progress with less data plane outage.

  0: https://github.com/ovn-
  org/ovn/commit/1dd27ea7aea40122c1edbff845e14abaa70c0413

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