[nova][kolla] questions on cells

Dan Smith dms at danplanet.com
Tue Oct 1 13:37:42 UTC 2019

Mark Goddard <mark at stackhpc.com> writes:
> The thinking behind upgrading one cell at a time is to limit the blast
> radius if something goes wrong. You suggest it would be better to roll
> all cell conductors at the same time though - do you think it's safer
> to run with the version disparity between conductor and computes
> rather than super- and cell- conductors?

Yes, the conductors and computes are built to work at different
versions. Conductors, not so much. While you can pin the conductor RPC
version to *technically* make them talk, they will do things like
migrate data to new formats in the cell databases and since they *are*
the insulation layer against such changes, older conductors are going to
be unhappy if new conductors move data underneath them before they're

> Here is the line that made me think policies are required in
> conductors:
> https://opendev.org/openstack/nova/src/commit/6d5fdb4ef4dc3e5f40298e751d966ca54b2ae902/nova/compute/api.py#L666.
> I guess this is only required for cell conductors though?

No, actually more likely to be the superconductors I think. However, it
could technically be called at the cell level so you probably need to
make sure it's there. That might be something left-over from a check
that moved to the API and could now be removed (or ignored).


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