Comment 0 for bug 1691195

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

If an instance has had an attached volume volume-updated twice in a "round-trip" - ie, volume-update $vol1 $vol2, then volume-update $vol2 $vol1 - it cannot be live-migrated.

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

1. Create two iscsi volumes.
   # cinder create --name test_vol1 --volume-type iscsi 1
   # cinder create --name test_vol2 --volume-type iscsi 1

   (--volume-type iscsi isn't mandatory - in my devstack environment there is no iscsi
   volume-type, but that doesn't stop me from reproducing this bug)

2. Boot an instance.
   # nova boot --flavor 1 --image $imageid --nic net-id=$netid testvm1

3. Attach one iscsi volume to testvm1.
   # nova volume-attach testvm1 $test_vol1

4. Do volume-update to swap volume to 2nd one. (1st time volume-update)
   # nova volume-update testvm1 $test_vol1 $test_vol2

5. Do volume-update again to swap volume back to the 1st one. (2nd time volume-update)
   # nova volume-update testvm1 $test_vol2 $test_vol1

6. Live migrate instance to other compute node.
   # nova live-migration testvm1

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

Live migration succeeds.

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

Live migration fails with:

Apr 27 10:32:14 multi9h-3 nova-compute: File "/usr/lib64/python2.7/site-packages/libvirt.py", line 1939, in migrateToURI3

Apr 27 10:32:14 multi9h-3 nova-compute: if ret == -1: raise libvirtError ('virDomainMigrateToURI3() failed', dom=self)

Apr 27 10:32:14 multi9h-3 nova-compute: libvirtError: missing source information for device vdb

Environment
===========

This has been originally reported [1] in Red Hat OSP 9 (Mitaka) and is reproducible on devstack master as well.

Additional information
======================

There are two things going on here.

1. When performing the volume-update, the libvirt driver calls virDomainBlockRebase without the VIR_DOMAIN_BLOCK_REBASE_COPY_DEV flag [2], meaning the device XML changes from <source dev=/dev/isci/lun> to <source file=/dev/iscsi/lun>. This is a problem because /dev/iscsi/lun isn't a regular file, and causes the above error, except you need the "round-trip" volume-update to trigger it. Why? Because:

2. The serial number isn't updated when doing volume-update, and there's a bit of live-migration code [3] that checks for serial numbers before updating the XML. If the serial numbers don't match, the XML isn't updated, and libvirt doesn't notice that /dev/iscsi/lun isn't a file.

[1] https://bugzilla.redhat.com/show_bug.cgi?id=1446446
[2] http://libvirt.org/html/libvirt-libvirt-domain.html#virDomainBlockRebase
[3] https://github.com/openstack/nova/blob/master/nova/virt/libvirt/migration.py#L158