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[cinder] lvm_mirrors option seems to be broken


On Tue, Jul 30, 2019 at 10:45:20AM -0600, Mark Lehrer wrote:
> Final suggestion: on my system, lvm seems to work best with the
> "--type raid10" option.
>
> Perhaps a cinder.conf option for lvm_type=raid10 would be useful?
> raid1 too?  I am seeing this warning from lvcreate quite a lot, so
> something needs to change fairly soon:
>
> WARNING: Log type "mirrored" is DEPRECATED and will be removed in the
> future. Use RAID1 LV or disk log instead.
>
> Thanks,
> Mark
>

Looks like we have a couple of bugs.

Mirroring does not appear to be supported with thin provisioned volumes, but
our code does not currently check if the two are used in combination. At a
minimum we should recognize that and give a nice informative warning message in
the log. Since we now default to trying to create a thin pool unless explicitly
asked for thick provisioning, this could be more of an issue. We should also
put a warning in the help text for the lvm_mirrors config option.

The `--mirrorlog mirrored` option is deprecated. Based on your experiments, we
may want to just change that to `--type raid1`, or maybe better `--type
raid10`.

I'm not sure all the permutations of possible configurations here, so it might
take someone a little time to validate the correct args for each situation. But
definitely looks like this is an area that needs some attention.

Thanks for digging in to this and reporting your results.

Sean