online_data_migrations docs don't mention using --config-file to run the migrations per cell db

[Matt Riedemann]

This came up in the mailing list while answering some questions about when/how various cells v2 and database related commands get run:

http://lists.openstack.org/pipermail/openstack-discuss/2019-October/009937.html

Recent change https://review.opendev.org/#/c/671298/ was added to the upgrade guide to mention that you can use the --config-file option with the nova-manage db sync command to migrate the cell database schema per cell database, in most cases that being cell0 and cell1. The same is true for the online_data_migrations command since that does data migrations for both the API DB and cell DB, and you would need to run it per cell DB using the --config-file option with a config file whose [database]/connection is configured for a given cell, e.g. cell0 or cell1.

So I think the CLI guide should probably be updated for nova-manage and the upgrades guide like in https://review.opendev.org/#/c/671298/. For the CLI guide, it might be useful to just have a generic section about using --config-file per cell database for commands that require a cell database but don't have a kind of --all-cells option like the archive_deleted_rows and purge commands.

[Matt Riedemann (mriedem)]

Also related:

(11:48:31 AM) mgoddard: mriedem: is it possible to run ODM for a cell DB before other cells have been upgraded?
(11:49:10 AM) mriedem: possible? sure
(11:49:11 AM) mgoddard: just wondering about the upgrade one cell at a time scenario, should I include the data migrations as part of that cell's upgrade, or all together at the end?
(11:49:20 AM) mgoddard: advisable?
(11:49:50 AM) mriedem: i'm not sure
(11:49:52 AM) mriedem: dansmith: ^?
(11:50:07 AM) mriedem: i would think it's ok to do it per cell upgrade
(11:50:15 AM) dansmith: mriedem: mgoddard at the end
(11:50:27 AM) mgoddard: dansmith: ack, thanks
(11:50:47 AM) dansmith: mriedem: do you mean target the cell and do data migrations just within that cell?
(11:50:56 AM) mriedem: i think that's the question yeah
(11:51:14 AM) dansmith: I mean, I guess that's okay, but since they don't need to be run until later, I would just do them later I guess,
(11:51:22 AM) dansmith: and if the api is all upgraded first then those migrations would be fine
(11:51:33 AM) dansmith: so I guess either way, but I'd just do it later personally
(11:51:37 AM) dansmith: mgoddard: ^
(11:51:50 AM) mriedem: mayhap we should be capturing some of this in the faqs https://docs.openstack.org/nova/latest/user/cells.html#faqs