Unable to swap between RBD volumes using virDomainBlockRebase

[Lee Yarwood]

Description
===========

$subject, the use of libvirt's virDomainBlockRebase API [1] currently means that we can't swap between volumes that do not present a block device or file on the compute host.

This is additionally blocked by the following conditional in Nova's libvirt driver:

https://github.com/openstack/nova/blob/f454e1dec9580abf4605e071bdd678a40f492a49/nova/virt/libvirt/driver.py#L1854-L1865

This is however possible with the virDomainBlockCopy API [2] when the domain is using the new -blockdev framework for detailing how disks are attached to the guest.

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

Steps to reproduce
==================

* Launch an instance with a RBD volume attached.
* Try to swap to another volume.

Expected result
===============

Success using virDomainBlockCopy.

Actual result
=============

Failure using virDomainBlockRebase as no block device or file is presented to the compute host.

Environment
===========
1. Exact version of OpenStack you are running. See the following
  list for all releases: http://docs.openstack.org/releases/

master

2. Which hypervisor did you use?
   (For example: Libvirt + KVM, Libvirt + XEN, Hyper-V, PowerKVM, ...)
   What's the version of that?

libvirt + QEMU/KVM

2. Which storage type did you use?
   (For example: Ceph, LVM, GPFS, ...)
   What's the version of that?

Ceph

3. Which networking type did you use?
   (For example: nova-network, Neutron with OpenVSwitch, ...)

N/A

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.opendev.org/696834
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=fafbc182f9179c16b89c45d02544d4582e0a1194
Submitter: Zuul
Branch: master

commit fafbc182f9179c16b89c45d02544d4582e0a1194
Author: Lee Yarwood <email address hidden>
Date: Sun Dec 1 16:32:47 2019 +0000

    libvirt: Use virDomainBlockCopy to swap volumes when using -blockdev

    Previously virDomainBlockRebase [1] was used by swap_volume to switch
    between volumes presented to the compute host as block devices or files.

    As outlined in the virDomainBlockCopy [2] documentation this command is
    actually a superset of virDomainBlockRebase in our case:

    > This command is a superset of the older virDomainBlockRebase() when used
    > with the VIR_DOMAIN_BLOCK_REBASE_COPY flag, and offers better control
    > over the destination format, the ability to copy to a destination that
    > is not a local file, and the possibility of additional tuning
    > parameters.

    As such we can switch to virDomainBlockCopy and expand support for
    swap_volume outside of just host block devices and files.

    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

    Closes-Bug: #1868996
    Change-Id: I8e8035dcf508f5215bba9b7575c5c6abfe41da31