[openstack-dev] Considerations when adding Nova config options

Mark McLoughlin markmc at redhat.com
Mon Aug 13 11:44:20 UTC 2012


Hi,

I just noticed we've added a new 'defer_iptables_apply' configuration
option to Nova:

  https://bugs.launchpad.net/nova/+bug/1034401

The default value is False but AFAICT there is no reason why users would
*not* want to change this to True.

Also, the docs for the option does nothing to help users decide whether
they should change the option:

 +    cfg.BoolOpt('defer_iptables_apply',
 +                default=False,
 +                help='Whether to batch up the application of IPTables rules'
 +                     ' during a host restart and apply all at the end of the'
 +                     ' init phase'),

I can only see one justification for this being an option - perhaps
we're unsure about the reliability of this "defer" mode and plan to
switch to defer mode by default in future and remove the option. If so,
it would be nice if the docs said something like:

  Set to True to evaluate this new experimental behaviour which is 
  likely to become the default behaviour in a future version.


Another similarish example is where we're considering adding an
'ignore_attach_device' option:

  https://bugs.launchpad.net/nova/+bug/1004328

Again, the default is False but AFAICT there's no reason users of the
libvirt driver would *not* want to change this to True.

And again, the docs give no guidance to users about when they might want
to change it to True.


We now have roughly 500 configuration options in Nova. That's absolutely
insane. Especially if some of them are set by default to some known
broken value and we don't have docs to tell users to change them.

I'd like us to start rationalizing configuration options - removing
some, marking others as "experimental", adding more helpful guidance to
the docs, etc. - but I'd really like if we could make a better effort to
not make the problem worse in the meantime :-)

Cheers,
Mark.




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