[Documentation] Migration in cinder, lvm-rbd example is not correct

[Vladislav Belogrudov]

This bug tracker is for errors with the documentation, use the following as a template and remove or add fields as you see fit. Convert [ ] into [x] to check boxes:

- [x] This doc is inaccurate in this way: ___lvm <-> rbd migration for live VMs just works ___
- [ ] This is a doc addition request.
- [ ] I have a fix to the document that I can paste below including example: input and output.

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-----------------------------------
Release: 20.1.0.dev57 on 2020-06-02 19:31:34
SHA: 3e068b5ce0990a576b55fea952319884c5c0ed6d
Source: https://opendev.org/openstack/cinder/src/doc/source/contributor/migration.rst
URL: https://docs.openstack.org/cinder/latest/contributor/migration.html

[Sofia Enriquez (lsofia-enriquez)]

Greetings Vladislav Belogrudov,
Do you want us to update the documentation just to be sure, or do you have another goal?

Would you please tell us how you reproduce the scenario?

Cheers,
Sofia

[Vladislav Belogrudov (vlad-belogrudov)]

I will correct the documentation.

Interesting enough, it did not work before :)

Migration from Train + Ceph Luminous worked only outwards. So cinder "retype --migration-policy on-demand rbdvol1 lvm-1" worked, but "retype --migration-policy on-demand lvmvol1 rbd-1" did not. There was a fix for Ussuri to allow migration into Ceph, it requires

libvirt >= 6
qemu >= 4.2

The bugfix:

https://review.opendev.org/c/openstack/nova/+/696834/
..
To allow swap_volume to support RBD volumes we also need the domain to
use the recently introduced -blockdev support within libvirt >= 6.0.0
and QEMU >= 4.2.0. New MIN_LIBVIRT_BLOCKDEV and MIN_QEMU_BLOCKDEV
version constants are introduced and used to determine when to switch to
the virDomainBlockCopy method of moving between volumes.

[1] https://libvirt.org/html/libvirt-libvirt-domain.html#virDomainBlockRebase
[2] https://libvirt.org/html/libvirt-libvirt-domain.html#virDomainBlockCopy

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/c/openstack/cinder/+/839872

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.opendev.org/c/openstack/cinder/+/839872
Committed: https://opendev.org/openstack/cinder/commit/7ee223837ec99bf85d8a4b220457e6c1302acc3a
Submitter: "Zuul (22348)"
Branch: master

commit 7ee223837ec99bf85d8a4b220457e6c1302acc3a
Author: Vladislav Belogrudov <email address hidden>
Date: Fri Apr 29 10:45:37 2022 +0300

    Fix example of failed migrarion for LVM->RBD

    Migration from and to RBD devices works for in-use volumes since
    Ussuri release. Also as of now a migration of in-use volumes for
    non-active VMs is prohibited for any backends.

    Related-Bug: #1673090
    Related-Bug: #1868996
    Closes-Bug: #1969913
    Change-Id: Id639c412a6f5ad24ab9c80416c56c1d99b3f104c

[OpenStack Infra (hudson-openstack)]

This issue was fixed in the openstack/cinder 21.0.0.0rc1 release candidate.